What rank do you have to be to attend all the courses in the light fighters school at Fort Drum?

That's going to depend a lot on what course it is to you want to attend.

That's a tricky ones as you have several schools that fall under Light Fighter but are Rank Specific. Most can be attended as a Specialist E4 but E5 Sergeant is your best bet for all of them. My first time through Pre-Ranger was as an E3 private First Class. Now its called Mountain Leader Combat Certification Course (MLCCC). Mandatory for non-ranger tabbed NCO's between 95-03. Not sure now.
Hope this helps. You can always go down and ask, most instructors aren't dicks when not on the range and are more than willing to help.

It was preferred that a soldier be E4(P) or up when I was there but that was nearly 20 years ago