I read a book back in 1998 that was a teen science fiction novel with a black cover. I distinctly recall a pair of large emerald green cat eyes on the front cover. The story involved a girl and several friends, and there was some sort of wooden chest in a large house/school that they would use as a portal to another realm. Hoping someone can help me out...
Could it be "Under the Cat's Eye" by Gillian Rubinstein?
I'm not sure, but the description sounded similar in several respects. And Amazon says it was published in 1998, so the timing also fits.
The title includes "cat's eye", and one of the covers is black and has a large greenish cat eye. It's only one, not a pair, though.
the main character, Jai, is male, but he has two friends, "Hugo" and "Seal". "Seal" is a girl and is quite important to the plot. According to a Prezi by Henry T, at the end of the story,
"Seal finds out that she is the lost baby, not Jai."
the story takes place at a boarding school, and according to Thomas Myer's review
the awful chest in the hallway where the baby died [...] turns out to be a portal between parallel universes.
The cover makes me think of Catmagic by Whitley Striber but the story sounds completely different. Any chance you have the cover and story mixed up?
Takes place in Maywell, New Jersey, a sleepy little town, the forces around it quite, until in a laboratory George Walker completes his first experiment in reanimation: He kills an animinal and brings it back to life.
Human subjects are next.