Penzu.com is a website diary. It is free. Also it has a pro* choice.
Your students can use it easily on their tablets, androids, ipads and blackberries.
*"What features does Penzu Pro have? Penzu Pro includes a ton of great features like: mobile access (native iPhone and Android apps), multiple journals, Penzu Post(journaling via email), themes, importing, exporting, and so much more.