Before you do this, you must have insurance in place. These ‘get rich quick’ schemes rarely work out and you will need to be experienced enough to be sure you can look after somebody else’s dog properly. As said, I rather doubt you’d get many people, if any, willing to entrust their pet to somebody who is still at school. Wit respect.
Whether or not you declare it (and you should) is the least of your worries with this idea.