ScootPad is a great resource.  I like it because educators can set a grade level for the student and add specific assignments for each student.  Parents can also log in and give "homework" to students allowing for parents and teachers to be on the same page.  ScootPad offers a limited free service but even purchasing a subscription is very affordable (around $100 per year for an entire classroom).  
Kids get immediate feedback on their answers which can be a good or bad feature.  It's nice that students can see how they have done but if they have any sort of math anxiety, having their wrong answers highlighted in red can be a bit frustrating.  Overall, this program is user-friendly and a great educational value.

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