That's strange. That's not right if they don't allow water in class. You should bring a bottle anyways and if they complain, then just say that you need to stay hydrated because it is beneficial to your health and if the teacher doesn't like that, then you will complain to the office.
.. Or just claim that you feel sick/ you're diabetic.
or just splash the teacher, then chug the bottle.
theres a reason for it, though. vodka looks like wter..