What are some good, simple games you could play with a bunch of 14-18 years old special needs teenagers?
That depends entirely on what they like and what their needs are. I can’t really tell you good games without knowing the teenagers in question. All “special needs” tells me is that someone decided that these teenagers should be in a segregated program rather than integrated with non-disabled peers.
You should take into account the very real possibility that kids that age might not be especially interested in playing simple games. A good percentage of teenagers aren’t, and being classed as “special needs” doesn’t necessarily change that.
There are tons of websites that have suggestions for games to play with people of various ages. (Including adults. Don’t ignore suggestions meant for adults). I’d say look those up, see if there are any that seem like the folks you work with might enjoy, and try them. And then, if that doesn’t work, do something else.
But also, ask them. If they’re people who have expressive language, ask them if they know any good games, or what else they’d like to do. If not, make suggestions and see how they react. Respect their communication and preferences.
No one that age should ever have to play a game they don’t want to play.