I remember as a child (so late 2000s, early 2010s) watching a television series (on Nickelodeon I believe) about a group of three high schoolers, led by their principal, who formed a team that fought various aliens on Earth. I don't remember any of the plots, other than an episode where Kelsey Grammer (Frasier) voiced a sponge. Yeah, it was a bit weird. Does anyone remember this show and have any fond memories of it?
It sounds like The Troop. Wikipedia lists Kelsey Grammer as playing a superintelligent sponge.
The show revolves around Jake, an average teenager who wants to create his own comic book series and is enlisted by his school counselor Mr. Stockley to join "The Troop" a secret society that protects the world from monsters and creatures. Additional squad members include his fellow students, Hayley, the most popular girl in school and Felix, the school's main nerd. Teenagers are chosen for the troop because their minds are more accepting of the strange things in the world, but they are old enough to understand the responsibilities that are entailed. It is shown that once you grow too old monsters become more frightening. For some it is a small fear, for others it becomes completely terrifying to even look at a monster's picture. The pilot was shot in 2008 and the show itself has been described as "Men in Black at a suburban high school".
Dr. Cranius (voiced by Kelsey Grammer) – An extremely intelligent sponge who attended Harvard, he is considered a monster and had to be contained. At the end of episode 24 Cranius is shown being heavily locked up in a real monster cell.