Questions for data analysis of a class room
Very difficult question for someone to answer as ultimately your examples and many questions contain certain constraints you won't get from your sensors.
For example:
Your first question is predicting "optimum" time to turn on AC.  who defines optimum?  Optimum for the classroom at full capcity, half capacity?
Your second question looks like it one, requires a map of the classroom and where the sensors are, two you won't be able to predict 7 sensor values from 2 sensor values, I don't know why you would want to do this.
Your third questions is a social sciences problem.  Define "light needed to be comfortable".  I imagine you would get 150 answers if you had 150 students.

Your scenario lacks context/comparison.  Let's say you had sensors outside the classroom for your various data points.  Then you could do things like, can I predict the temp inside the room based on my other room and external data points.

Finally, yes machine learning can be used with your wireless sensors.