The Jekyll and Hyde Phenomenon

This morning Adam and I went for a parent-teacher meeting at William's school.  Mrs. L raved about our boy. He knows most of his upper and lower case letters, and about half of the sounds they make. He can write out the numbers between one and ten all by himself.  He spends most of the time building things out of Lego and always has a new creation to show the class during share time.  When he does get around to putting pencil to paper, he creates wonderfully imaginative drawings with fantastical stories to go along will them. He's independent, cleans up after himself, and is happy playing with his friends or by himself.  I was pleased with the meeting, not because any of this is a surprise, but because Mrs. L seems to get William and genuinely appreciate him.  She also gets the sense that he enjoys his time at school.

So why, then, is it now our routine to have temper tantrums every school day? Sometimes he fusses about school itself, other times he invents other reasons to throw a fit.  Today he wanted to bring a toy that shoots missiles and we asked him to choose a more school-appropriate toy or even just leave the missile at home.  Giant meltdown.  He became way too upset to pick something else.  I ended up dragging him to school screaming.  I pulled him by his jacket, his back pack.  If I let him go he ran in the opposite direction.  When he made his legs go limp I had to pick him up. Then he punched and kicked me in the face.  He bit my knuckles to get me to let go.

This isn't separation anxiety.  He isn't being bullied.  There's no reason for this other than torturing his parents on a semiweekly basis.  Mrs. L says that the routine might stop when he's had a break for the summer and Jadzia goes with him in the fall. Anyone have a quicker solution?