The poem seems to picture someone, perhaps a teacher, taking attendance and being pulled back into the memory of school. The first line calls school "a time of lead," which suggests heaviness: dull, grey, hard to carry. The second line sounds like a vow. "Sworn to answer present" turns the simple word "present" into something almost solemn, as if showing up were a duty pressed on a child. It also plays on the idea of being present in the moment, which the speaker may not have felt back then.
The last two lines soften the mood. Children's grammar is compared to "an old shop / found in a new place," a lovely image. It suggests that kids speak with odd, charming constructions that feel like antiques, old words and habits turning up somewhere unexpected. The speaker seems to move from weary memory toward a gentle, curious fondness. The overall mood is quiet and reflective, with heaviness easing into surprise and a little warmth.