I worked for a local running store for a long time. We rarely hired HS kids for a few key reasons:
1) Their availability is terrible, especially if they are a runner (i.e., doing track and/or XC). They hardly ever work a closing shift during the week because of practice/school and can hardly ever work a full shift on the weekends because of practice/meets/social events. The only time they could work was during the summer when we already have plenty of help.
2) If it's only an internship/summer work, then they are really only good for the register or an extra hand to help set up running events, expos, or field marketing events. It is not worth the effort to fully train the HS student on all of the shoes, insoles, recovery, nutrition, accessories, and apparel if they are only going to be on the floor for 3 months. By the time they actually learn anything they will be back to school.
3) We generally looked for people who had at least some customer service experience. LRS are all about the customer service, otherwise it would just be a Foot Locker, Dicks, or online store. Most HS students had zero customer service experience, so they were passed up immediately.
4) Most customers don't really take suggestions from HS students seriously and I completely understand. If a 35 year old father of 2 has questions about shin splints or training to BQ or nutrition questions, they are more likely to actually listen to someone who has run a marathon before or has done Boston or has a few seasons of life under their belt. Not a HS student who is just parroting what they've been told, but who lacks actual experience.
Just a few reasons off the top of my head....