If you want to express belonging, you can only use "de":
- El equipo del colegio. (We know what school you're talking about.)
- El equipo de mi colegio. (A bit possessive if we already know which team you're talking about, but fine.)
- El equipo en el colegio. (It doesn't necessarily express a sense of belonging or that it represents the school, just that it was formed there.)
- El equipo en mi colegio. (Same as previous, but this time we know it's your school for sure.)
