I was thinking about this today following a meeting with school. ASD is a complex, multifaceted issue. It spans difficulties with sensory processing, movement, social interaction, language, working memory/literacy, self regulation (bowel, bladder, sleep/wake, attention). As such, a child on the spectrum can meet the criteria for other conditions e.g. Dyspraxia, dyslexia, SPD.
The difference is that there is no exact picture that fits every person with ASD. As someone else said on MN- when you have met one person with ASD, you have met one person with ASD.
The picture changes from day-to-day. And if you are assessing a child with relation to school; then other factors should be considered by a competent professional e.g. is it the start or end of term (tiredness), is the child compliant at school but unravels once home (good insight as to where greatest demand is), is today a school track suit or gym day (might prefer to wear shirt and tie, or might not, might prefer gym, might not).
OP there has been some great advice and insight from others here. I would echo whoever said about keeping a diary; note activities, what allowances you made, what warning you give about activities, how he reacts to unflagged schedule changes, mood before/after school. I feel that parents underestimate the level of support/adaptation they provide for the higher functioning ASD and this becomes 'invisible' even though your child would fall apart without it. And that could make or break your case for diagnosis.
For me having that label has been useful- though I have emphasised to school that on no terms is it to be used in public. Labels are easy to apply- very hard to remove.