for assessment - You can set a student a math problem and record their working out as they go. Record two students solving a problem and her the language they use
and to help struggling students with concepts in maths - record yourself doing a lesson on the app and the student an play it over and over.
I like Educreations because it is simple to use as it doesn't have heaps of buttons etc. The limitation with this one is you can't save creations/lessons to the camera roll. Much like Show Me they get uploaded to cyber space. By creating and logging into the same account on each iPad you can easily share what you have made and what your students have made with each other.
The great thing about Explain Everything is that you can upload to the camera roll and then publish creations. The trouble is it has a lot more buttons and is confusing for younger students.
There is another app that has an interface more like Educreations but can be uploaded to the camera roll. Vittle is a bit expensive but the free version is worth checking out.