It is a tough call.
At all the SM races I have been to the AM class was stacked with talent and the Expert class had few riders. There does need to be some type of an organized system of progression up to the Expert class. In SM the age classes are usually the largest classes and they could probably be split up if enough riders sign up.
AMA MD MX used a points advancement system. 1st place was 5 points down to 5th place was 1 point. 15 points moved you to B class. 30 points moved a B rider to A class.
In MD there was (still is?) a D class that was for inexperienced riders. They could ride it the D class for 1 year but 1 win and you were moved to C class.
The sad fact is that SM does not currently attract enough riders to necessitate an A, B & C class but it does not help attract and keep new riders when really fast riders are Cherry picking the newbies.
I wish I could make your 1st race and give a hand but I am not going to be able to make it up.
When is the next race?