GCSE students (i.e. under 16's) are forced by law to stay in school! hence as you say ''most GCSE students would struggle with the ATPL modules" most of the students in my class at 16 could not give a rats backside about what was being taught - its not really the same with ATPL students who have invested time and money and really want to succeed in becomming pilots.
if you think human performance, operational procedures, IFR comms, VFR comms, airlaw (amoungst others) are at degree level then you clearly havent tried studying for advanced aerodynamics 2 or perhaps structural analysis on an engineering degree course?
actually GCSE Physics and Maths (certainly A-Level) was a lot harder than say Radio Nav or GNAV. I know students in my ATPL class who prepared for the exams simply by using the database questions - i had to work much harder for my GCSE's and A-levels
Thats my view anyway mate - im not trying to put anyone off a ''pilots studies degree''