Question

1. Which adjective combination is correct?
Todos los estudiantes en esta clase son ____ pero demasiado_____.

Answer 1

You need the pair of plural adjectives which modify the plural noun "estudiantes". I only see one pair meeting that criterion.

Answer 2

Yeah, since the sentence is talking about "estudiantes," which means "students," (plural)
your choice should be the one where both of the adjectives are also plural.