TS - a Masterclass is very much participative, and furthermore pretty informal, and in a way it's like a school/college class (but think more like a tutor group if you are using an educational comparison).
You normally either sit round one big table, boardroom style, or more normally grouped in tables of say 2-4 and the 'master' will conduct the class from the front and walk round the audience, talking you through what you are nosing/tasting with Q&A.
What normally happens is that a distillery or brand ambassador will take you through a 'flight' of their products, for example Glenfarclas 10, 15, CS '105', 21 and 30 year old expressions. If you are lucky, ie at a good one, you may also get some special stuff or perhaps to taste their new-make spirit or specific cask samples, from say a bourbon or sherry cask.
The one's I've been at have been in the region of 1-1.5 hours, but this will depend if the Masterclass is a one-off or part of a bigger event or exhibition.
They differ very much from a normal 'tasting' where you get a couple of drams from a rep behind a stall and as such a Masterclass would normally give you a chance to explore in depth the range from one particular distiller or brand.
So very much recommended