Can't seem to talk to my crush

So, as of the last 2 months, me and my crush have been talking on ***. We are eachothers #1 best friend on the app and have small convos and send eachother doodles. However, when we are in class I can't seem to talk to him. The other day I finally talked to him in person and complimented his sweater but since then I'm back to my old ways of basically ignoring him. Does anyone have any tips on how I can start a conversation with him? We have class together but sit at different tables.