I'm gonna have to second, third, fourth and fifth Full Metal Panic, FMP: Fumoffu and FMP: The second raid. The story line is great. I am willing to speculate that there will probably be some more stuff animated from this series in the future since the novels seems to be ongoing still. It's funny, there's some good romance thrown in with the action (not enough to hinder the action and certainly not too little to be transparent). Its the kind of romance which drives the plot and the action. Very good series.
As far as other school based anime goes you could try Kekkaishi for a bit of it, they go to school and by night protect it from spirits (a tiny bit of romance though i've only seen till the 7th episode). This one is still ongoing.
Gunparade march: Short but amazing, about high school students in the military, fighting for man kinds survival. Saw it, loved it, wished it was longer. Romance is dominantly present. This isn't so much of a mech/fighter anime as a ... well a not a mech fighter anime. Mostly focuses on romance, with a bit of the aspect of human suffering and loss. Though it is relatively happy, so you won't get all depressed watching.
Gunparade Orchestra: Though i haven't seen this one i want to, just can't find a place to get my hands on it
. Made by the same people that made Gunparade March, i hear it's very similar, with different characters, more comedy and 14 more episodes.
Hikaru no Go: If you like Go and even if you dislike Go, alot of it happens at school and school tournaments. To be honest this anime didn't really do it for me, but it's not because it was bad, more because it didn't suit my tastes.
Sorry if anything i suggested has been mentioned (as i'm sure it has been with so many replies). Good luck filling your second 1TB HD .. ^_^