It should be clear that using se for sie in written language is incorrect, except for reproducing spoken language.
However, in oral language, se for sie (3rd person singular or plural) or Sie (formal address) is quite common and I cannot assign it to a special dialect. It is colloquial speech that can be heard from Berlin to Munich to Cologne. For example:
Na, hörnse mal! (Hören Sie...)
Hamse das gesehen? (Haben Sie...)
Kommense mal her! (Kommen Sie...)
Kommen deine Eltern zu Besuch oder kommense nicht? (...kommen sie nicht)
Kommt deine Schwester zu Besuch oder kommtse nicht? (... kommt sie nicht)
In my understanding, it is essential that the pronoun se can only be used when it comes after the verb. It is then a clitic with reduced schwa vowel.
And sometimes this schwa even gets omitted, see the comments below.
I don't think that your grades get worse for using the colloquial form se; at least not as long as the task wasn't explicitly to use formal German.