1. I'm in classroom now.
2. I'm in the class now.
3. I'm in class now.
4. I'm in my class now.
I think for sentence 1, it should be I'm in the classroom now. Sentences 2 and 3 are fine.
Sentence 3 is wrong because of the missing article 'the'.
Am I correct?
As for 1, you're right - there should be a determiner, like the, preceding classroom.
As for 2, you normally use a determiner, such as the, to talk about a group of students that you're in together, e.g. I'm in the (same) class with my cousin Michael now.
As for 3, it's all right to use in class without any determiner; here, class means a lesson; if you are a teacher, and you say I'm in class now, it means you are during a lesson, giving students a lesson now; if you are a student, and you say I'm in (English) class now, it means that you're during a lesson, attending a lesson that a teacher is giving now.