"You are the most intelligent one in the class."
Translation:Tú eres la más inteligente de la clase.
When you're using a superlative - the most or least something, so oldest, prettiest, least attractive, whatever - the construct is to use de there. You're the most intelligent of the class, the prettiest of the girls, oldest of the children, etc, would be a closer direct translation there.
Inteligente is one of the adjectives that doesn't change for gender (unlike caro or cara for expensive), so either el or la is correct depending on the subject you are applying the adjective to, in the case of this question there is no context, therefore either should be accepted.
What does inteligente modify - subject or the predicate nominative.
Ella es la más simpático maestro de la escuela. (If the school has both male and female teachers?
If we write Ella es la más simpatica maestra de la escuela, does it limit the teacher group to only the female teachers in the school.
First, you say "la maestra más simpática", not "la más simpática maestra". Second, I'm pretty sure that saying "ella es la maestra más simpática de la escuela" doesn't limit it to only female teachers. (Please correct me, native speakers.) Third, I'm also pretty sure that using "ella" and "maestro" would be incorrect. (Though, I guess you can say "ella es la más simpática de los maestros en la escuela" - she is the nicest of the teachers in the school - though that's not a literal translation of the sentence, so it would likely not be accepted.)