my classmates can reply but not me

I suppose you're talking about the "quote reply message". Try signing out from Teams manually (top right corner) and the right-click on the taskbar icon and select "Quit". Sign in and have a look. If still not there, you can check manually for updates (top right corner too). Most updates are being released backend but sometimes that is necessary. For the question why, it's difficult to explain really as the update process is very intermittent.