Is this Zixx Level One (2004-2009)...?
An alien named Zixx Phunkee-Zee and her partner Flanngo crash land on Earth while investigating a lead on behalf of The Network. Once there they discover that the planet isn't as insignificant as it seems. For one thing, it's home to an ancient pocket dimension maze known as The Keep, which just so happens to operate a lot like an Earth video game. It's also where the evil Hargokk Empire is planning its return. Led by Deeth, they seek to resurrect the evil alien warlord Oncolon from deep within The Keep and take over the universe.
Fortunately for Zixx and Flanngo, they receive help from local human children in investigating the secrets of The Keep and fighting off Deeth and his cronies. Best friends Adam and Griff help the duo in Level One, later being swapped out for Riley, Megan, and Dwayne in Level Two and Level Three.
As mentioned in the synopsis, the protagonists are an Intergalactic Network agent (who looks like a human girl), her partner (who resembles a small dinosaur), and two school boys from Earth. The scenes set on Earth are live-action, whilst the scenes set in the Keep (a cyber dimension) are 3D-animated.
You can view the scene where the four protagonists meet around the 11:33 mark in the video below.
The school principal is featured as an antagonist. You can view a scene with him at around the 15:19 mark.