Can't connect to classmate's host world

For our project we require someone to host a world but each time we try to connect to each other's worlds a message pops up saying "unable to connect to world" and then brings us back to the main menu. We have tried hosting different worlds with still no luck joining each other and we can't connect through I.P. address. However this has just been a problem for the past couple days and we have tried joining each other's worlds previously from home and it worked. The world doesn't look to be corrupted either and other classmates have not run into the same issue. What do we do?