Following are five barriers to entry that you must overcome if you want to do much school assembly speaking work.
The decision makers at schools are hidden. It is the Principal, the Counselor, the Student Activities Director, a student organization leader, a student, etc.?
The school assembly times/dates are moving targets. Most schools don't do the same assemblies at the same time every year and most schools work off of a different template than other schools.
The decision making process is a moving target. The time of year for this changes and is different from school to school.
The budgets are moving targets. Its not like the schools have a set "training budget" they pull funds from. Some schools use general funds, some use Perkins money, some use federal drug education money, some use federal grant money (like the new bully prevention grants), some use student organization appropriation or fundraising money, etc.
The decision makers at schools are experienced. Many of the decision makers at the schools are involved and active. This means that they have probably seen 5-10 speakers in the past year that they could possibly bring in to speak at their school. The VAST MAJORITY of my assembly work has been from word-of-mouth marketing
So, how can you overcome them? Well the list of answers is long. But, here are are three quickies...
I will say that the foundation is creating relationships within a small cluster of schools and going from there. The market may be huge, but you have to take a small approach.
Do student conference speaking. You can speak at one student conference and get exposure to hundreds of schools. Not necessarily the decision makers, though.
You also have to be extremely good to get work via mord of mouth!