Official documents can be found on the EU website. In fact, there is no such thing as a “tourist” Schengen visa, the main distinction is between a short-stay (no more than 90 days in a 180-day period) visa and long-stay (national) visas. But you must always specify the purpose of your journey on the application form.
In fact, the standard application form (annex 1 of the visa code) includes a “Study” checkbox and both the visa code and the Handbook for the processing of visa applications and the modification of issued visas also discuss at length the type of supporting documentation that must be provided in this case. In particular:
- for journeys undertaken for the purposes of study or other types of training:
(a) a certificate of enrolment at an educational establishment for the purposes of attending vocational or theoretical courses within the framework of basic and further training;
(b) student cards or certificates of the courses to be attended;
All that should be enough to establish that studying or taking classes on a short-stay Schengen visa is officially allowed and completely above board. Now, if you pretend to be a tourist and submit a complete itinerary while the real goal of your trip in fact is to attend a course, it could be grounds to refuse the visa (if caught, obviously).
Of course, the short duration of stay (even compared to the UK's six-month general visitor visa) severely limits the type of classes you could possibly attend.