Writing a reference for a high school student should be considered a privilege, as you have been entrusted by the student to represent her to a prospective college or company. When writing a reference you'll want to include the basics: how long you've known the student and in what way. Furthermore, you want to follow any specific guidelines set by the organization, such as formatting, listing your professional qualifications or using a specific reference form.
Supplement the Student's Application
Whether the student is applying for admission to college, an internship or a job, make sure you expound upon her achievements instead of reiterating what your reader will already know. Remark on the student's character that might not be visible in the classroom. For example, if this student is a standout tennis athlete and also helps out by coaching at a local training center, you could talk about how well she works with kids. The bottom line is that you want to supply insight and information that can't be attained simply by reading the student's application.
Focus on Relevant Intangibles
Not that concrete achievements aren't worth mentioning, but you have the opportunity to communicate essential character details about this student that make her an ideal candidate for the position. You could talk about the challenges she's faced, and the resourcefulness she's shown in facing those obstacles. Perhaps she embodies the quirk of being abnormally well-organized; this is your chance to show that in the light of a positive attribute. If possible, expound upon qualities that have made her such an asset to her high school, qualities that will also make her a valuable asset to the school or job to which she's applying.
Less is More
Having paragraph upon paragraph of complimentary things to say about a student is a good thing, but an applicant reference letter should be concentrated and concise. Instead of listing every act of selfless compassion this student has exhibited, choose the one that best represents the point you're trying to make. Remember, your reference is only one part of her application packet, and you have a better chance of getting the reader to remember one salient detail than 10 interchangeable ones.
Offer an Anecdote
Anecdote is an effective method for conveying a sense of character about a person, as well as a means to ensure your reader will remember what you have said. Furthermore, application readers read massive amounts of information, even when evaluating a single application packet, so breaking the monotony of dry facts with a story can be both refreshing and pleasant for your reader. Tell a brief story that shows the student exhibiting qualities that you want to communicate, that way the reader can judge her actions for himself and see her the same way you do.
Proofread for Perfection
Even though you aren't the focus of the recommendation, you want your reader to accept you as a competent and educated reference. Be sure to write your reference on your letterhead -- unless you are using a prescribed form -- and proofread your letter several times for errors of punctuation, usage and mechanics. A grammatically-sound reference allows the reader to forget about you as the writer and instead focus on your recommendation of the student.