A high school student cover letter would come in very hand to all the high school
students and recent graduates that are usually aggressively looking for a job. Approaching as many places as possible and filling out applications is good practice and it's definitely in the numbers to get a job. Having a little on your side can help open up your opportunities.
One way I found to increase my opportunities was to always have with me a resume and cover letter to turn in with my application. Better yet, if I had the time and opportunity to take the application back at a later time. I would put together a cover letter specifically for that job position.
The easiest way to stand out with a high school student cover letter is to focus your letter and attention to what you can bring to the position. You won't be able to rely on your experience but being young and hungry can be just as appealing.
Focus your letter on the following order of importance.
1. Why you want a job.
2. Energy you have and would bring to the position.
3. Ability to work well with people of all ages and backgrounds.
4. Ability to learn quickly and implement what you learn with little supervision after learning.
5. Not afraid to ask question and learn from your piers.
6. Availability to work the hours the position requires.
When I applied those points to the high school student cover letter I was using, I found that my lack of experience wasn't as much of an issue and I was offered the opportunity to sit in and start learning the position I would be taking.
In my case I was taking over the position of someone moving up in the company, so it was easy to learn and still have someone to go to afterward.