Teasing is normally made as a joke, it can be a bunch of small jokes put together or slighlty annoying them on a weird subject or personally. But harassing is againt the law, this could be touching someone, hurting them and many other bad things. Iv'e been harassed and i can assure you that it is very truamatic and whenever you think back at it makes you feel weak. and this is because harassin cant be stopped, no matter who you tell or scream or shout because sometimes people call you a liar and you seem weak to yourself and others.
He realizes that one positive thing about not living in Berlin anymore is that her friends are not around to bully him for being the smallest boy in his class. He comforts himself with the idea that when they return to their home in Berlin, he might be big enough that they won't make fun of him anymore.