I'm not exactly sure if this is the right category to put this under, but I think it fits! Schools (middle schools, high schools, colleges) - invite your alumni back! Most likely, they will be willing. If not, then you don't want them there anyway. Your alumni were in your students shoes at one point. They understand your specific school culture, the demands, the needs, etc. Invite them to speak about their journey to spark inspiration and to contribute to the college-going culture.
Middle school is a crucial time, especially in NYC, because it's your high school grades and activities (and the rigor of your HS if you're in NYC) that start counting toward your college application. As much as a teacher might tell them that, they are higher authority. High school would obviously be a great time to do this because the college process starts. College is also valuable to motivate students to succeed, graduate, and to think about a career. One important thing - make sure that the alumni you bring back reflects the population of students you are inviting into the room. For example, invite alumni of color to speak in front of the students of color at a college.
Choose alumni that you are proud of and you know will be willing to stick around and talk to students afterwards. Alumni who are willing to leave their contact information and are open to speak to current students if they choose to contact them.
This is not a lasting mentorship solution. However, it's a really easy one to execute. It's the first step to exposure, especially to people current students can relate to. It begins to instill the value of networking and a community. That even people outside of their teachers or family are here to support them. To show that they are part of a larger community that the school has worked to maintain. A strong alumni network can be extremely valuable.