When people perceive themselves as similar to others, greater liking and closer relationships typically result. In the first randomized field experiment that leverages actual similarities to improve real-world relationships, we examined the affiliations between 315 9th grade students and their 25 teachers. Students in the treatment condition received feedback on 5 similarities that they shared with their teachers; each teacher received parallel feedback regarding about half of his or her 9th grade students. Five weeks after our intervention, those in the treatment conditions perceived greater similarity with their counterparts. Furthermore, when teachers received feedback about their similarities with specific students, they perceived better relationships with those students, and those students earned higher course grades. Exploratory analyses suggest that these effects are concentrated within relationships between teachers and their "underserved" students. This brief intervention appears to close the achievement gap at this school by over 60%.