We have raced at many tracks... Some are neat and organized, some are a free for all.
For two years I ran the mini and quad track at Kenworthy's. Practice was always by class (4 str quads, 80s, 50s, 60s then 2str quads). There was a reason... Don Kenworthy LOVES to soak the track. Less watering, more racing. The 4 strokers would fling the mud... 80s usually did well in the mud and by the time the 60s and 50s were on the track there were good lines and ruts. The two strokers would then "smooth" the track back out and you were ready to race.
Now, Kenworthy's never runs an "open practice". And, only the last race of the year do they run the mini jrs (80cc 7 to 11) on the big track. Several other tracks that we frequently went for "open practice" would run 50s through 80 beginners for 30 minutes then 80s "advanced" through open for an hour. It worked well. But, we would NEVER attend an open practice (more than once) that was a free for all.
So what is my point... If you are under 18 you have a parent with you. That parent should be held fully accountable for the minor's riding ability and safety. If the practice is not organized, or they are running 50s with the big boys and quads, the parent should pack their kid and go elsewhere.
When my Tyler was on 60s, I liked to see him run with the bigger bikes. Had he been hurt, it would have been my fault, not the track owners.
TMax