Some things to consider:Schools need to work in schedules. Usually classroom sessions are about 45-55 minutes in length. Figure a typical school day goes from 8:30-2:30 for the most part, with some buffer on either end of that. Lunch time--11:30-1:00 is not ideal of course and anything after 3PM simply won't work.
It is best in this environment for you to be fairly flexible in your delivery. We are not necessarily looking for a canned presentation.. more along the lines of show the students something you are working on, and then let them ask some questions.. maybe you even pose some kind of (fairly simple) problem to them.
It does not have to be formal-- in fact informal is usually better. A facilitator will help to work the technology and keep the conversation flowing... you just need to be willing to ask questions and answer their questions around the work you do.