Which one is correct? 他们是我的老师们/他们是我的老师
These four are all authentic expression while the first one is the best choice if you are introducing your teachers to someone.
他们是我的老师
他们是我老师
他们是我的朋友
他们是我朋友

1.老师 or 朋友
Both are acceptable. But it's usually more natural to call your teachers 老师 instead of 朋友 because it sounds more polite and formal.
2.我的老师 or 我的老师们
It's more natural in the Chinese language to say 他们是我的老师 instead of 老师们 even though &quot;teachers&quot; make more sense than &quot;teacher&quot; when understanding the sentence in the way of English grammar. It's hard for me to provide a reason for that.
3.我的老师 or 我老师
我 and 我的 both mean &quot;my&quot;. The omission of 的 makes it more colloquial. It's common to hear people say 我... to a friend. But again I guess 我的老师 would suits better because it sound more formal.