Did you mean "textbooks"? The sentence is missing something, "it" would work here.
"I do not understand it when several students attend classes without textbooks". Here, "it" refers to the condition of non-understanding the writer has when the event of several students attending classes without books occurs.
A case could be made that there is nothing wrong with the sentence. This would indicate that the writer lacks understanding (an inability to learn) when several students do not bring their books to class, though how this could happen is beyond my understanding. The nearest I can get to this concept is, "I do not understand when drinking a bottle of whiskey".
I have just been reading the last thread you started in which you were told twice that you must start every sentence with a capital letter. You failed to do so several times in that thread and you have done it twice in this thread, in just three sentences.
In post #1, you should have used a capital for "What is wrong with this sentence?"
As we said before, your profile shows that you are an English teacher. It is extremely important that you write correctly on the forum because learners will read your posts, see that you are a teacher and will copy your constructions.
Please note that it is not appropriate or necessary to use terms such as "sir" or "madam" in your posts. Firstly, it's far too formal and secondly, you don't know what sex the people here are. Gillnetter could be a woman so "sir" would have been completely wrong.