Believe it or not, hot tea does the trick when my voice starts to go. Also, if you have allergies, that can take a lot of strength out of your voice.

Rest your voice at lunch and breaks. If possible, find an empty classroom where students cannot find you. Eat your lunch or drink your tea in peace. The break will do your voice good.

Those are all great responses. Absolutely - warm up your voice in the morning. take some time at lunchtime to refocus your voice.

Humming lightly (with an easy relaxed throat) feeling the vibration strongly on your lips is a good warm up. Also play with making lip sounds like motor boat or a BBBRRRRRRRR sound as if you are cold...the strength of those sounds with the breath it takes to produce them will lift your voice.

AND yes - design exercise time when the audience talks and interacts and you can relax...