Most dressage barns that are looking for a "working student" (at least ones in the states that are worth going to) are looking for someone who has experience riding PSG and higher. Even then, it is not a "free ride" and a lot of times you are still expected to pay board, but your training fees are comped.
You're also treated like crap and expected to be riding 10-20 horses a day. You often are last on the lesson list (often don't get a lesson on your own horse) and the head trainer will just yell stuff at you across the arena when you're on random horses. On days off, a lot of times you are still expected to be on the property, and many times still riding.
Yes, you can learn a lot. But you are working your ass off (one girl I know dropped 20 pounds in 4 months, and she was skinny to begin with). It is not for the faint of heart, or those that can't handle the young horses, unruly stallions, horses with rearing/bucking/bolting issues, etc..
I looked into a position a few year ago and I've ridden up to 3rd/4th, worked with young horses, developing horses, etc.. They would hardly consider me and said that they'd still be charging $1500/mo even though I would be working every day. Then I looked into getting a position in Germany and that was a laugh! You're up at 5-6 doing chores, you eat breakfast for 5-10 minutes and then you're riding young horses all day (each person had 5-15 horses assigned to them) on top of tacking up horses for your supervisor, then you do evening chores, and then you eat supper and get to go to sleep. It's a 10 on 1 off rotation? I think? But you need your own vehicle to leave the property.
If you really want to learn dressage, then you have to fork out a lot of money. Being a working student is not the best way to go. You do a lot of work riding other people's horses basically, and you don't get paid and you rarely get lessons. Keep going how you are going, you have a good horse and it seems like a good coach. Keep moving up that way and instead of riding for these big name trainers, train with them.