Question regarding having a 21 day long stay in the Netherlands first, then studying in France [duplicate]

It's OK to enter the Netherlands with a French visa (see Should my first trip be to the country which issued my Schengen Visa? for the rules on short-stay visas and Can I visit Schengen countries on a long term French Visa? about long-stay visas). That's possible even if you enter the Netherlands directly from a non-Schengen country (i.e. not going through France first), see German national visa (Type D), but entry through another Schengen country? for details on this particular issue.

Furthermore, you will need a French visa to stay six months in France (a Dutch Schengen visa is not enough). These visas sometimes have additional requirements and you need to apply for it from Iran in advance so you need to start with this application ASAP in any case.

Where things get a little complicated is that the rule quoted by the Dutch consulate is the rule for Schengen visas but that's not what you will get from France. Instead you will need a long-stay visa and I am not sure whether you would get one valid from August 2 if you only have documentation pertaining to a 6-month program starting on August 24 (especially considering the fact that going over the 6-month threshold would change the type of visa).

So if you get a French visa that does not cover your intended period of stay in the Netherlands, it might be necessary to apply for an additional short-stay visa as well.