You are not in overstay if your visa expires in the weekend, no problem to go on Monday and you shouldn't have any problem asking them to correct their mistake. On samui they are quite picky and moody so don't forget to be extremely nice and polite 😬
We recommend to our students to go to Penang, K. Barhu or Vientiane, no problem at the moment. The tourist visa CAN'T be converted into a ED without leaving the country if you study at a private school