Is this correct? If I wanna say there are 15 students in this class room do I say "この教室には学生15人がいます"? is it correct? I feel like something is missing between 学生 and １５人 should I add の to make the sentence sound normal?

で particle on any action Do で only use on just "eat/ate in (place)" or can be use in any action as well like jogging, walking, running etc.

まだ/未だ usage Where do I put this? Is this a topic, subject? Or do I put it in the middle of a sentence like "私は日本語未だお勉強です"?