"Please don't go to school yet."
It gave me 「行かないで」and then 「行か」「ない」and「で」separately... Why?? XD
Yes, either one is accepted. Clicking on them separately or clicking on the pre-populated one. It's there so you know what you translating.
I don't think duo acknowledges へ existing as a particular outside one lesson.
What is the difference between まだ学校に行かないでください and 学校へまで行かないでください?
(The latter was accepted for me, but I don't understand what the difference is.)