If I have a young child that has been playing school tennis for a couple of years and he/she is interested in entering a tournament, how do I go about determining what skill level they are? I have no idea what the rankings mean and I can't find it on the USTA site.
If you think you know what level she is, sign her up for a tournament of that ranking and see how it goes. Or you can meet some usta players and play with them and see how she does. If she has a usta coach or trainer, they will know what level she is.
The USTA website is kind of hard to navigate... trust me, I know. But if your child is under ten, then you should try out quickstart tournaments. I don't really know how high his level is, but quickstart is the easiest (I believe) but there are higher options if that's what you want.
You might want to try an 10U L3 tournament. These are played in a round robin format and can gain some experience for your child. L2's and L1b's should probably come later.
Before 6/7 years old it is better to play tennis as a game (differences between january and december are significative, the development is not regular in early-years).
Around 10 years old is possible to take part to tournaments to understand the level of the player.
Before USTA website it is better to follow a tennis school were professional trainer that works daily on young player can give you an idea n the level
Based on you referring to something as "school tennis" I'm guessing it's not any kind of USTA sanctioned junior tennis against other junior players that have USTA rankings - so my answer would be it depends on the child's age. If they are 10 or younger they are eligible to play quickstart tennis (smaller court, foam balls, etc.). If they're older, try and find a tennis facility that has teaching pro on staff that is familiar with the USTA players and tournaments in the region you live in and can direct you on how to find out the skill level the player is at the type of tournaments/competition they should be looking to play in. A site like this should be able to help in your search: http://usptafindapro.com/default.aspx/MenuItemID/654/MenuGroup/Find-a-Pro.htm