From the official Facebook page of the Al-Zahra Secondary Girls School in Jenin:
It sure sounds like they are saying that Israel shouldn't exist.
(I looked up about 12 schools, all girls, on FB. Most of the boys' schools did not have FB pages. Of those schools these were the only political posters I found, although one school did have a photo of the funeral of a terrorist last week calling him a martyr.)
Clueless Westerners love to point to surveys saying that Palestinian Arabs want a two-state solution, but you won't find a single poster advocating peace in their school websites.