Been to Schwantz's school too.
This comes up periodically, and the last time it did I wrote this:
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This just came up on another board, and here is my take. I've done Spencer's 5 times, Schwantz once, STAR 5-6 times, CSS 3 times, and DP many many times, and IMHO Spencer's school has no comparison as a complete package, for street or track riders - no question. If you're a competent street rider you'll walk away a vastly better rider for street and track. It's expensive, but if you are serious about riding and can afford it, you will definitely think it is money well-spent.
STAR is a very good package too, and an excellent value for the money, but perhaps best done after you have a bit of track experience from another school, because they go through a lot fairly quickly.
CSS is very good at teaching specific things that help you, especially for track riding, but they're a bit short on explaining the why's, so unfortunately a lot of people seem to walk away from CSS missing the point. It's fine for first-time track riders, just keep an open mind and think of their drills as drills, not necessarily as the final word on riding techniques.
Schwantz's school is a lot like a two-day STAR school, with similar topics and style, but with the added experience benefit of Kevin Schwantz (and Road Atlanta, which is *really* fun track). Probably best done after experience with another school first, again because they go through a lot fairly quickly, and it's expensive enough that you'd want to maximize your benefit.
If you're brand new to the track and mostly focused on street riding, you might consider CLASS - I have not taken it, but I have heard good things about it, and I recently signed up my wife for her first track school at CLASS.
My first was DP Safety School, which is a fine starting place too. The "A" and "B" groups are basically no passing-rules track days, but the "C" group for first-timers is a real school with a classroom, drills, etc.
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Basically these are real *schools*, which track days are not (even though some nowadays have "instructors", which generally means local fast guys who show you around and who may or may not be competent teachers). They take their curriculums seriously, and you walk away having learned a great deal.