I would recommend to book at least a month prior to the date you would like. And I would suggest at least 2 months if you are looking to book in the Fall, which is my busiest time of year and those slots fill up very fast!

I would recommend to book your seniors' session the summer before their senior year, or August/September of their senior year, at the latest.  Their senior year is packed with so many obligations, senior events, college application deadlines, that it is really smart to get them done before all of that starts.  And most schools require you to submit your casual image to the yearbook by November 1st.

Yes, $100 retainer fee is due at the time of booking. 
This will ensure that your time/session is held for you.  And it will be applied your total session fee.