What is this?
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You're almost certainly correct - but L. pilicornis has been recorded from the Oxford area, and a school is just the sort of place it could turn up. At that size I guess it could also be L. peregrinus?
BMIG website: www.bmig.org.uk (centipedes, millipedes & woodlice)
BMIG Newsletters ~ www.bmig.org.uk/view/resource/bmig-newsletter
BMIG Bulletins ~ www.bmig.org.uk/view/resource/bmig-bulletin
I don`t think it is L.pilicornis but most probably, as indicated by others, L.forficatus.
picture not sharp enough to see tergites on back in detail but the general "jiz" is of L.forficatus.
Lat/Lng: 51.7538, -1.2984
OS grid ref: SP485063