By a 'small school' how small do you actually mean?
Like a Betta sorority, they will get along better in larger numbers, or you are best off with just a breeding pair. More on that later...
I have 7 now in my 90lt tank and am upgrading to a 48 x 18 x 24, that's 300lt or so and should hold the number the fish I've got even at full adult size. I went for 18 wide because it will be a fully decked out planted tank.
Depending on the tank size you go with, I'd get a group of 6-8 and let them grow up together, some will eventually pair off and if someone is getting picked on you can just take them back and get more. I've added ones the size of 20c coins in with my breeding pair when they were about 3.5in in body length (fins not included) and while the female was unhappy and chased them away, things settled down and they are now fine.
Sexing them can be difficult even as adults and sexing them at the sizes you see them in an LFS is impossible. I wouldn't even worry about it. Buy a group and remove the 1-2 trouble makers you'll likely get.
If you want a breeding pair, buy 4 and let them grow up. You'll get two pair off (it's obvious when they do) and you can sell the others if they are getting picked on or disturbing the breeding pair.
Temperament varies as much as Bettas do. You get the ones who don't get along with anybody, you get ones who is every bodies best friend and the odd loner. They are all pretty intelligent fish and will recognize their owner/feeder, lol. I can't really say they school, but that's likely my tank size. They do hang out in groups, though.
By the time they get to adult size they will easily fit tetra and rasbora in their mouths. That said my pair are big enough to eat the rasbora in there at the moment but they don't. That may change though and if it does I'll just switch to a larger tetra like widow or diamond. I don't worry about it, everything is kept well fed. I did lose a few Espei and my Maculata to a feisty shark though.
Standard tropical fish temperature of 26 is fine. Mine are kept in soft acidic water because that's what most of the fish in there prefer, and so do the plants. They are fine in neutral though.
Hmm, I think that's everything? I have no connections to breeders. I just buy whatever pretty fish happen to land in the shop. :)