Would "Is Rosa your weird teacher, Sam?" be like this: هَل روزا مُعَلِّمَتَك غَريبة يا سام؟
Close. It would be like this: هَلْ روزا مُعَلِّمَتُكَ الغَرِيبَةُ يا سام؟
You need to add the definite article الـ to the adjective غريبة, because "your weird teacher" refers to a specific person, therefore we need to add the definite article to what specifies that person which, in this sentence is the adjective "weird".