Mom’s advice about good manners can help you get accepted to college.   No joke!  Thank you notes (and emails) are effective ways to convey your appreciation and reinforce your interest.
Many colleges track applicants’ communications with admissions representatives, and may use that information as an indication of your interest in attending.   “Yield” – the percentage of accepted students who enroll – a measure of desirability – is considered important at many schools.
Alumni interviewers volunteer their time to meet with students and then report back their thoughts and observations.  A timely, and well-written thank you email, can enhance an interviewer’s or an admission rep’s impression of a candidate, as well as reiterate the applicant’s interest in the college or university. Properly thanking those who recommend you is also important.
Teachers, coaches and counselors can spend considerable time writing thoughtful recommendations, and are sometimes contacted by an admission officer for additional insight.
In addition to doing the “right thing,” exhibiting good manners makes a positive impression, which can enhance your candidacy.