I think that Keroko-san and others raises a good point. I don't think that the SSS and Tenshi would ally with each other so easily. Most of it stems from how I believe their point of view on the whole war is. So here is my thoughts of how the SSS see Tenshi and how Tenshi see them.
SSS POV
Tenshi is their
enemy number 1. For Yuri she is the closest thing to God in this world or perhaps in gaming terms that
one boss that keeps them from advancing to the next part of their quest. Technically speaking, before this episode for me the SSS whole point of existence is beat Tenshi at something and hope something will happen.
I believe that Yuri and by extent the SSS battlefront did not plan on what will happen if Tenshi is beaten...
And who can blame them, Tenshi I think gives the appearance of someone that is invincible and they are painfully aware that they can't truly beat her so they'll take any victory that they can get. Tenshi also doesn't display any emotions or reactions to their antics which may irritate them a bit since it gives an impression that they are unimportant. Look at Hinata in the baseball game, he wished that Tenshi at least said something or threatened them. They may subconsciously want some acknowledgment from Tenshi which is one of the reason that they pester her so much.
Beside that, judging from what their previous reaction to Tenshi they do not believe that Tenshi can be beaten or else what happened like Tenshi losing her position is just a minor setback and Tenshi will still be their main enemy or at least remain in an antagonistic role.
In summary, even with the subsequent revelation that it seems only 2 members have yet realized the SSS can't easily buddy-buddy with Tenshi because to most of them the fight with Tenshi is on a
personal level. For most of the SSS, Tenshi is seen as the Worthy Opponent, Rival, or even the Big Bad. In fact, I would even dare say that until now, Tenshi
just by existing and doing her job gave the SSS battlefront meaning - giving vindication to their belief and most importantly a clear opponent to struggle against with.
Tenshi POV
On the other hand, I believe that Tenshi's view on the battlefront could not be more opposite. Tenshi's fight on the SSS battlefront is on an
impersonal basis - I'm just doing my job kind of way. She will only stop the SSS battlefront if they are breaking the rules and even then she shows a great amount of leniency. In fact, judging from the previous episode, the SSS battlefront does more damage to themselves than Tenshi ever did to them.
Point of View between SSS and Tenshi on what really happened:
PS: Ordinary text denote what the SSS believed happened and italics denote what happened in Tenshi pov.
PPS: Most of this is heavily relying on conjuncture which is not solid fact feel free to contest anything written here.
Episode 1 - Tenshi walks toward them presumably to stop them, SSS battlefront brings the big guns and Tenshi activates force field and just stand there. SSS battlefront waste ridiculous amount of ammo and managed to lift the ticket from NPC.
Result: Operation success for the battlefront.
(
Tenshi gives a mandatory show of effort to stop the battlefront since they are breaking the rules. SSS battlefront begins to shoot at her, Tenshi doesn't mind and just stand there, they are wasting ammo on her that they can use to attack the other students. 
The SSS lifts the ticket from the NPC but I don't think Tenshi really minds that, they give great morale to the student body in exchange for meal tickets that the students can pick outside if they're hungry. I really don't think that Tenshi cares if this operation succeeds heck she doesn't even try to stop it beyond showing up.
Result: Tenshi made them waste ammo which they can use to disrupt school and give student body free concerts.
Episode 2 - SSS tries to replenish their ammo thus, they go to guild. They get caught on their own trap and destroy their own guild in the process.
Result : Eerr... SSS battlefront success because they managed to escape and can build anew. 
Tenshi discovers the location of the new guild. She shows up to stop the creation of dangerous firearms. The 2 remaining battlefront members start shooting at her she puts up a forcefield. One goes to hand to hand combat and she responds in kind. The SSS decides to blow up the guild and cause Tenshi to fall to a fiery inferno. That has to hurt. Tenshi seems to not hold a grudge for that so its okay
Result: Tenshi managed to destroy guild even if it is temporary, the production of dangerous weapon at least slowed down.
Episode 3 - SSS holds an announced concert without official permission as a distraction while they infiltrate Tenshi's room. SSS finds some earthshaking revelation and lose their vocal lead.
Result: The operation is a success but they have heavy loses in Iwasawa-san and their mission statement has been delivered a massive blow.
Tenshi is supposed to take off the poster for the unauthorized concert but decided against it. Suppose to stop the GirlDeMo concert but, haven't done anything to stop it while she was there, instead the teachers stop it. Decided to listen to Iwasawa's final ballad before she disappeared. Heck, Tenshi haven't done a single thing in this episode to stop SSS. Most of her actions here are halfhearted which makes me wonder about her motivations?
Result: I have no idea... maybe Iwasawa-san's disappearance?
Episode 4 - SSS decides to illegally enter a baseball tournament. They were doing so well until they were taken out by the Student Council team. Hinata is conveniently placed on the exact same position that will make him solve his regret. But, thanks to the bakaness of his teammate is saved from it.
Tenshi assembled a baseball team to take out the SSS illegally entered team. and acts a coach to them. Easily defeats the good SSS team but struggled against the competent idiots. Either the Hinata team is really that good or Tenshi has another agenda considering that Hinata is placed on the same exact position that may have fulfilled him somehow.
Result: Tenshi's team managed to beat the illegal team, but Hinata did not dissipate?
So, in conclusion from looking at Tenshi's pov, I don't think that Tenshi even considered the SSS battlefront as an enemy, heck she might even be helping them in her own way. Most of what we know of Tenshi is still shrouded in mystery even if Episode 5 showed many interesting facts about her.
Of course, even if Tenshi does not have a motivation to refuse to ally to the SSS meant that Tenshi would agree to the alliance. We must remember that Tenshi is portrayed as someone that is on more on the Lawful Good alignment and if you consider it the SSS battlefront is breaking the rules. Beside this, we don't know what is the relationship of Naoto and Kanade, maybe they could be friends for all we know. At the very least they are coworkers being on the student council together.
In short, if the SSS and Tenshi ever permanently make an alliance together. A lot of paradigm shifting would be needed. It won't be happening soon unless Naoto is really that
bad...