Where screenwriters learn the form and logline their screen ideas.
PROCESS OF ELIMINATION
I agree with the comment above.
I would also add that 'transported' seems like a strange word choice to me. Transported by what? A bus?
I assume they are transported magically, and if that's the case I don't think it would hurt to say 'magically transported' or something similar. Without an adjective it just seems out of place.
The 'agent' part also sticks out to me. It just doesn't evoke an image in my mind. Again, I think a adjective is the fix. (or just be more specific as suggested by Richiev)
I believe as written, your language is too generic to properly captivate the reader.
What will help is specifics
1) A group of high school students--This is as generic as it gets. First you need a lead character, however even if you decide it should be a group, be more specific, A group of high school football players, a group of high school mathletes, for instance.
2) are transported to a castle-- A castle? A medieval castle, an English castle, a dark and scary castle. Is the castle on earth or in a different dimension?
3) when a promise is broken --What promise?
4) with a agent from hell.-- An imp? a powerful demon? The devil in disguise?
5) The Students must find a way out-- If the agent-from-hell can transport them to a castle, how is escaping going to help? He can just re-transport them. This seems like a flaw.
6) while defending themselves against many strange and deadly creatures--This isn't too bad, in fact I would just cut the strange part, it isn't needed "While defending themselves against the castles deadly creatures"
Remember this is what first drafts are for, to give a foundation that will be re-written and improved.
If you can be a little more descriptive and specific I believe your next draft of this logline will properly reflect your story in an interesting way.
Hope that helped, Good luck with this!