We built ClassCalc with this exact thought in mind.
Our lockdown is more of a "handshake" between teachers and students - teachers allow students to use their devices during class, and students agree to have their devices locked down to ClassCalc only.
Students can leave Test Mode at any time - and all that happens is that the teacher is notified the student left Test Mode. This way, students have complete control over their devices, and teachers can rest easy knowing that devices in their classroom are being used for educational purposes only.
Feel free to check out our demo here!