In a perfectly competitive market, if Jason raises his lawn-mowing service price to $20 from the going rate of $12, he is likely to lose customers as they can easily find other providers charging less for the same service.
If Jason, a high-school student, is working in a perfectly competitive market for lawn-mowing services and the going rate is $12, then if he tries to charge $20 for his services, he is likely to lose customers.
In a perfectly competitive market, there are numerous providers offering the same service, leaving no single provider with pricing power. Because customers can easily find other competitors providing a similar service at a lower price, they have no incentive to pay the extra cost, regardless of Jason's experience.
The law of supply also suggests that if Jason charges more than the going rate, the quantity of lawns he is hired to mow could significantly decrease.