Ghost Hunt, D. Gray Man, Baccano!, Saiunkoku Monogatari, Kyo Kara Maou, Himitsu - The Revelation, and Air Master.
I have a ton more I would absolutely love to see sequels for but these are some of the main ones!
Shounen Onmyouji and Ghost Hunt DEFINITELY need one!
I would also love to see sequels for:
Saiunkoku Monogatari (after the second lol)
Juuni Kokki
Densetsu no Yuusha no Densetsu
Hagure Yuusha no Estetica
Mushi-shi
Fruits Basket
Gakuen Alice
Hakushaku to Yousei
Kamichama Karin
Bamboo Blade...
If you haven't already you should watch:
Nurarihyon no Mago
Natsume Yuujinchou
Shounen Onmyouji (not a school one but it is a really good series!)
Blood-C (Maybe?)
Ghost Hunt
You might have seen most of these but, just though I would recommend them.
Omg, there are so many I want to become animes! And here is just some of them:
Sangoku Rensenki ~Otome no Heihou!~
Asaki, Yumeishi
Crimsom Empire
Real Rode
Beyond The Future -Fix The Time Arrows-
Souten no Kanata (might make a better anime then game lol)
Edel Blume (SUCH A GOOD GAME!!! Wish...