"After you sleep, you must go to school"
Translation:Ar ôl i ti gysgu, rhaid i ti fynd i'r ysgol
English is the odd one here. We say "to the bank, to the post office, to the airport"... Welsh is simply more regular here and also says "to the school, to the church, to the work, to the hospital". (US speakers use that last one as well.)
Review the tips and notes for the lesson "Simple Future": https://www.duolingo.com/skill/cy/SimpleFuture -- the section "Going to the usual place".
(Mods - perhaps a section on that can be added to https://www.duolingo.com/skill/cy/PastTens1 as well, which is where i'r ysgol and i'r gwaith appear to be introduced first?)