Sounds like the other moms pointed to good disciplining resources. You need enough discipline to get his attention. If that means "no soccer", than that stinks for him, but you are shaping his character which is more important.
He is still in he transition time, but something else may be going on here if it continues more than another couple of weeks and after you have laid down proper behavior expectations.
He is 5 years old. Many boys don't develop the fine motor skills to handle pencils and scissors, etc, but you said he likes the work. It could be that he is asked to do a lot more schoolwork than he would care to do- even though he likes it... he's going from very little or no work to real "school." ("Why Gender Matters" by Dr Leonard Sax covers differences like girls hear better than boys and how they learn... good information to avoid school burn out for boys pushed into academic kindergarten before they are ready. I was really shocked by how differently boys think from girls.)
Do the behavior issues occur at certain times- like lining up, recess, or after free play? My nephew had this obsession with being first and heaven help anyone who got in his way... We worked on dealing with change/ variations in his expections. (I guess this is best described in "What your Explosive Child is trying to tell you") My nephew is one of those kids who would melt down if you say you are going to Target and you go to the drugstore first. We've had to really condition him to surprises/plan changes.
He also gets in trouble at school. My sister was feeling down with his "noncompliant" reports but I nagged her to find out what happened before to set it off. We found when he thinks the work will be hard or he might look dumb, he would rather act up. Is he trying to look cool? Is he strong-willed and the teacher is a threat to his new sense of independent self? (In our case, it now appears my nephew is probably bipolar rather than ADHD which didn't really explain his behavior.)
Can you hang out near the school and be called when he is working up to an act of defiance? If they call you in you can address the situation, send him back because he is expected to do his work and let him know he will have to pay you for your time away from work. You can figure out payment after school. (amalgamation of Love and Logic and Explosive Child books)
It sounds like you are on the right track but that you need some more information about what is really going on. Plus give it another week or two to make sure he's had the opportunity to adjust.