Absolutely fantastic ponygirl story! The best I have read here in a long time! A few things disappointed me though. Something to keep in mind in case you ever want to revisit this story or something. I myself have posted "new and improved" versions of stories I already posted because I thought of ways to flesh it out and make the story better after the fact. Of course that was on Hentai Foundry and not here though.
For one, it seemed to be rushed near the end. It goes from training in the fields, to having their former coach join them (which served no purpose story-wise since it didn't change anything and the girl's didn't react at all), to being sold off in secret in the span of a few paragraphs. The former coach thing should have been used as an example to the girls of how effective his methods were. He should have shown off how much more athletic she was from before. That would help convince them to try more extreme methods of training.
Secondly, the girls voluntarily shaving their heads seemed to be weird. Why would they voluntarily turn themselves into ponygirls by shaving their heads at that stage of the game? Maybe an offhanded comment from the coach about how their hair was adding wind resistance beforehand would make their decision make more sense.
Thirdly, their training never got sexual or lewd. They trained in their clothes up to the end despite having liked the treatment from the boys. I'm sure the coach could have come up with some reason to have them train with fewer and fewer clothing in the secluded area of his farm/house. I expected it to happen during spring break but was surprised when it didn't happen at all.
This isn't a complaint but more of a suggestion. Instead of having the girls drop out due to bad grades, why not include some of the teachers in their training in return for better grades. Or tie the girls to a post and let the teachers have a go at them. Have them perform sexual favors in return for better grades.
