We were going out then I broke up with him not knowing what I would be missing then he started going out with a girl I wasn't really friends with so it didn't matter as much but then he texted my best friend and told her he broke up with the other girl to go out with my best friend and she said yes... what should I do... Ignore it?

So... he's your ex but you still like him? What part of that doesn't make much sense?

Sounds like jealousy to me... you cut him loose... he's a free agent now and can go out with anyone. Ignore it... it stopped being your business when you broke up.

Sounds like all of you are very young, what you do is concentrate on school work.

However, whatever happens with an ex is of no concern to you, you go no contact and move on. Whether you still like him or not.