So today, I discovered your battleground does not actively check for participants levels. This isn't a serious bug as you can only have a one level advantage. I joined a battleground as level 19 with a little bit of XP left before turning 20. At the fire in the start I prayed and leveled to 20 from the XP given and it never removed me. Obviously it's not a common occurring thing as the chances aren't that great and it's really not worth the effort if you wanted to seriously exploit this... but it's still something minor I would fix.