Not really. It wouldn't be normal for a parent to "pick up" a child at the *beginning* of the regular classroom period, when he didn't appear to be sick (as far as has been reported) when seen with his stepmother at the science fair.
The wording of the statement above has me wondering just a little, if maybe at least one teacher recalls *seeing* the two on their way out of the school together. THAT would certainly explain why, if any teacher or school official wondered about his absence and suggested may be a call home was in order, that the call wouldn't have been made. It would also raise some huge questions about the stepmother's possible involvement in the disappearance.
One little detail I haven't seen discussed is the outcome of the Science Fair itself. These fairs usually involve awards being given to some students' projects . . . and not others. If this was a geeky, science-oriented little boy, is it possible that he became upset when his project didn't win a ribbon, and ran off because of that?