You need to study a minimum of around 8 hours per week to get an ED visa (fewer for private 1-on-1 lessons). Missing too many classes will result in the visa being revoked.
If you're really lucky, you might get everything in time (I doubt it). However, you're lucky in that you can enter on a visa exemption—which gives you 30 days, with the possibility to extend it for 30 days at 1,900THB. So you don't have to lose your flight.
(You can still use you SETV, which is valid for 3 months. But leaving and re-entering Thailand is difficult at the moment. Especially with Omicron in the next months.)