Strip Designer is the perfect Comic Strip app. It isn't free, but the price tag is small in comparison to what you can do with the app - only $2.99.
Strip Designer starts by letting you select a category template and then choose how many windows you want to include on the comic strip page. You aren't limited to one page either - you can create an entire comic book if you wish.
Just tap in a window to add a photo. Once the photo is on the page, you can make edits to it. You can also add speech balloons, stickers, effect text and even additional windows.
Once the comic is complete, you can export it to Google Drive, OneDrive or Drop Box. This is a great tool to let student creativity shine! I would love to see Romeo and Juliet retold by students using a comic book.