Which schools help arrange pre-MBA internships?

Pre MBA internships are rarely arranged/facilitated by schools. Most people get their internships through their personal networks. The only case I can recall of a school facilitating an internship opportunity is a PE/VC summer internship for incoming first years at Kellogg. The internship is sponsored by an alums company and there aren't many placements.

Many large firms that recruit at top schools have workshops and a few internships over the summer. Many of these are geared toward diversity admits (URMs, women, military, LGBTQ) and are more 1-2 day "get to know the company" events rather than internships. I received a lot of notifications (oftentimes overlapping) about these opportunities from Wharton, Booth, and Kellogg last spring.

You have to remember that you're not really matriculated until you are officially enrolled. There are many people who commit to a school in the spring only to forfeit their deposit later in the summer to go elsewhere. While applicants are being admitted in the winter and spring, current students are looking for jobs and internships. Schools' career services resources are wholly dedicated to these students' efforts so there really isn't anything left to help a potential student find a pre MBA internship. Obviously I cannot speak for all schools, but this was my experience based on the schools to which I was admitted. Hope this helps.