When you are talking about something that actually happens, you can use as:
Rob got quite drunk at the party, as he usually does.
You are talking about hypothetical situation, so you cannot use as. as if is the correct form for hypothetical situation.
Rob acted as if he owned the place.
As if must be followed by a complete clause, including a verb:
"At the lessons" is not idiomatic, but it's also unclear what your sentence is trying to say. This sounds like a direct translation of a Russian idiom, that might not have the intended meaning in English. I can think of three possible meanings:
If you mean during the general time spent in an institution of learning (both inside and outside the classroom), ...
It is usually more natural and idiomatic to say "in lessons" rather than "in the lessons". In your specific example, the word during sounds better still:
Children call each other names during lessons.
That said, in a context where you have already specified certain lessons, it could be appropriate to use the definite article to refer back to them. For ...