Help with multiplayer

Hello guys im in kinda need of some help right now because im playing on a build server and im going to show my friends in school what i've built but i cant connect to minecraft servers in my school, I can play some single player and on lan worlds but when i try to play multiplayer it doesn't work and i need some help to play on the multiplayer servers so please i've already put minecraft in the firewall and it still doesn't work so please guys help me!

//qarekgaming.