Tangled FX ($1.99)
With lots of options and customizations, TangledFX is my go to app for creating stylized versions of my classroom.
Percolator ($2.99) Beautiful circles create an abstract version of life in my classroom.
Bokeh Lens ($.99) Quickly mask off an area to remain sharp and then blur the rest. Intensify the blur strength until the student is not identified. This works best if what you are trying to show is in the foreground of the picture.
Prisma App (Free) Prisma lets you take your photographs and apply filters linked to famous paintings. There are new options all the time and they come out looking quite artsy. Some of the filters are not stylized enough to really conceal faces so you may need to try a few.
Touch Blur (Free) It's a little creepy to blur out their faces, but sometimes that's just what you need.
Oil Painting Effect (Free)
And my favorite...
With a little app practice you can share beautiful images of the learning in your classroom, without actually using student faces.