Startup School 2018 - from Y Combinator is a free online course for startups.
We were in the first batch. The weekly syncs were great because we could ask a bunch of questions and get feedback. They also kept us accountable to driving out metrics week/week. We also keep in touch with our mentor, a year later. A few teams, including ours, got to participate in live office hours with Sam Altman. It was a great opportunity to practice speaking and get some exposure. We wrote about our experience here: https://canny.io/blog/what-youll....
We met several other cool founders and had a great mentor. The advice is super valuable.
20+ teams to 1 mentor is a bit much.
You'll be surprised with what you can accomplish in 10 weeks. I wrote about my experience in last year's Startup School here: https://www.producthunt.com/posts/startup-school#comment-484477
The mentors (mostly YC alum) and small group sessions with other founders are so valuable.