Image editing

Note that free apps often have ads or might be limited in some way. Where I know of the ads or limitations, I have commented on them. Ads are usually clickable and will take you to a website. There is no guarantee that the site will be appropriate for students.

$2.99 ea. (1-19 units), $1.49 ea. (20+ units), Lite version is free but adds a logo to the edited image
Add captions and speech bubbles to images. Images can be saved to the Photo album or emailed. 

Change photos to black and white and then select which areas of the photo you would like to remain in color. Ads run at the top of the screen.
FREE - Ads run at the bottom of the screen.
Picture editing, portrait processing, picture effects.

FREE (in-app purchase available for some extra label types.)
Choose a photo, choose a label type, swipe across the photo where you want the label to and then type the text. 9 different label types, with 4 more available as in-app purchases. Edited image is saved to your Camera roll. Images can be shared from there, or shared to Facebook or Twitter.

 FREE (in-app purchase available for some extra features.)
Add effects, borders, change exposure or colors. Edited images can be saved to the Photo album or uploaded to Facebook.
If you have problems using it, Adobe has a support forum here