If braiding mane...

Do I have to braid her tail for a schooling event?

If it's a schooling show, no.

You don't even have to braid their mane for a schooling show.

it doesn't matter for a schooling show. you should if you need the practice or if your horse needs the experience standing quietly while being braided.

No braids are needed for schooling.

No braids are needed for schooling, but it can be fun to do and good practice. The tail does not need to be braided if the mane is, but if the tail is braided, the mane must be as well.

No, but I've never braided the tail for any show. Perhaps when I start riding in "real" classes, but so far I haven't

Tail braiding is really uncommon for event horses. More of a hunter thing.