Initially for the student-interface...

Attributes on the 'Answer' text <input> box:

• required
• pattern = "-?\d {1,3}

  (\.\d*)?"

• title - hint = "I'm expecting a number"

• native support in recent Firefox, Chrome, Safari and Opera,
• it will degrade gracefully in older browsers,
• it will be configurable, so that it can be disabled (just in case - I don't think you'll want to disable).