DS has just done this course as a 1 year twilight subject (1 hour after school). The theory is not too difficult and much can be self taught. This free site covers the syllabus:http://www.astronomygcse.co.uk/AstroGCSE/New%20Site/IndexNew.htm
And this is the set text book:http://www.amazon.co.uk/GCSE-Astronomy-Guide-Pupils-Teachers/dp/0953634574/ref=sr_1_1?ie=UTF8&qid=1364486226&sr=8-1
However the controlled assessment is (as hermonmunster says) the problem. The two projects can be done at home for the data gathering and photo taking, but must be typed up under controlled (exam) conditions. 6 hours were allowed per project. The projects are marked by the teacher (does the online company do this?) and only moderated by the exam board.
If the physics teacher is keen try to persuade him to let a small group do it after school as a pilot.