Aside from some mild flirting and kissing, there is not much at all.

A robot's head gets knocked off in one episode.

In one episode, Principal Barrage loses his leg. He is a cyborg, and the leg was mechanical so once he got it back, he just attached it back on. No gore.

The blue tazelwurms often try to attack people, but no serious harm is ever done.

There was an episode where the main character got into a fight with the three school bullies. Nobody was harmed though as the fight ended abruptly.

The Serpent can be quite violent, and he usually has the intent of harming or even killing others.

In one episode, the main character says the word "Screwed".

Some mild insults like "idiot", "stupid", etc.

Two characters in particular (Chaz and Kimmie) often insult others, but it is done for comedic purposes and is not taken seriously.

Principal Barrage's appearance and strict demeanor may frighten younger children

Throughout the first half of the series, RadCircles (the main antagonist) contacts Lee (the protagonist) through the phone using a voice modifier and a shadow to hide his appearance, which may scare some younger children.

The series gets progressively darker as it goes on.

The second half of the series is quite dark as it involves characters experiencing near-death situations, getting kidnapped, etc