It's not uncommon at all for students to repeat a grade when they enroll in a boarding school. It's a widespread practice based on some fairly simple principles.
Underlying repeating a high school grade are notions that successful completion of a repeat year can strengthen your college application.
Chronologically or emotionally a student may not be as mature as he/she needs to be before working through the college process.
Some students started school a bit young; repeating a year puts more age and experience under their belts. Students grow and mature at different rates; some need more time to mature.
A student may have gotten behind or need to make-up poor grades. A student may want to build a stronger foundation before applying to college.
A student who is also a good athlete may repeat a year to develop physically, emotionally, and athletically before attempting to enter highly competitive collegiate athletics.
Students with strong abilities in the arts may use an extra year to improve themselves and build a body of work in support of their college applications.
An inter-relation of all of the above.
You might want to consider a repeating a grade if you find yourself with any or all of these situations.
Contrary to conventional wisdom, colleges look positively on students who successfully repeat a year. Colleges want to know why a student repeats a year and see evidence that the student reached his or her goals. Colleges appreciate the maturity and dedication that result from a successful repeat year.
Alternatives beyond boarding school exist if you need to consider other places and ways to gain extra work and experiences. Junior colleges or beginning your college career in less competitive environment both provide excellent growth opportunities that allow students to transfer to a more competitive situation. However, each of these settings provides less adult supervision than a traditional boarding school.