@johndoehf
There are many factors influencing human's behavior.
In fact, it's not rare to see bullying victim doesn't fight back.
In this manga, I think the author has explained it well in detail already that Tendo is afraid of them and doesn't want the matter escalates. He is physically weak and he doesn't know how to fight. Instead of solving the problem, he will most likely end up failed and got punishment in return if he fight back. That's why he chooses to endure the pain instead.
Tendo is also an external locus of control dominant type, and he has a low self-efficacy to solve his current problem.