What would happen if it was found out that a child was in the wrong school year/grade?

the best hypothetical I can come up with; OK imagine a man has a one night stand with a woman and uses no protection, then about 14 months later he does the same thing again (but this time uses a condom) then he hears nothing from the women until a few years latter when she turns up at his door with a child and tells him it's his and then leaves the child with him and runs away, the man assumes the child must be 4 years old as it must have come from the first unprotected time... but in fact the child is actually only 3 because the condom broke ad he didn't realise, operating under this misconception he enrols the child in school a year early and a result it stays a year a heard of where it should be for several years until eventually the truth is found out somehow but by which time the child is like 16 (actually 14 obviously) so what would they do? move her back a year? or leave her where she was?