App for Education

Hi - I am a teacher looking for a very simple app that would have black magnetic letters on a white screen that children can pull down and create words with. I do not want color for the letters. This is an instructional tool so the teacher must have the ability to select multiple letters to use prior to the activity beginning (i.e. several a's, e's. etc.). However, all of the letters of the alphabet should not be visible at once - only the ones selected in advance by the teacher. The child should be able to pull the letters down to create a word and then put the letters back up to create new words from the letters predetermined by the teacher.