You can give vitamins short period, but I wouldn't do it more than a week at a time, especially if they have other food with concentrated vitamins in it...
If the chickens are extremely stressed due to environmental factors such as being inundated with lots of new people you can try chamomile.
https://www.backyardchickens.com/articles/how-to-grow-chamomile-for-chickens-a-beginners-guide.78461/#:~:text=Furthermore, chamomile possesses calming properties that aid in,introduction of new birds or challenging weather conditions.
Other than that, make sure they have toys and things around to distract them, hiding places, lots of roosting options (logs or tree branches work well for this).  Those will all help with reducing stress.
Fair warning, if there are too many chickens in a cramped space the stress will continue to remain high.
Do you have any pictures of the school set up?