In German does saying you like someone have the romantic connotation as it does in English? For example does "Du magst den Lehrer." also mean "He has a crush on the teacher" or is it simply "He thinks the teacher is good"?
I think most would understand it non-romantically, i.e. whether they can stand them at all or whether they think the teacher is a good person.
The is Der in Nominative (masculine) and Den in Accusative (masculine). Here the direct object is the teacher which means use of accusative den - den Lehrer.
Streber is a noun, so I believe you need an indefinite article here: "Du bist ein Streber".