At work I am considered to be a nice, pleasant, cheerful person, very willing to help people, and considered popular with the rest of the team by my manager.
I have worked hard at that, especially as in the past I've had low self esteem, and have been considered insular by one manager who totally underestimated the effect of only having partial hearing.
I have chosen to be friendly to, but not overly close to, my work colleagues. I like them, but I've never mixing work and friendship again. Had a real issue once.
In my personal life, I am generally cheerful and pleasant to everyone, but I only have a couple of close friends. I think some people don't like me, but I really don't give a fuck. This is me, take it or leave it.
OP, you may indeed find it helpful to keep some distance at the school, I have seen it go sour for others, but being cheerful and polite actually has helped me to keep some distance. I can pass the time of day with some superficial chit chat that is effectively non-committal, and some people simply can't see that I'm in reality keeping some distance. It's the cheerfulness I think, it gets mistaken for being friends. Suits me because then I'll take my time in assessing someone before deciding whether I want to be friends.
This isn't about being two-faced. I'm simply allowing people to see one top level of the person I am before letting a few people see the more in depth bits of me. I'm still cheerful and pleasant when you get to the deeper parts of me. 