A fascinating and very vintage story about a teacher seduced by a devious student as an act of revenge. It's hard to know if this film is supposed to be a comedy (I found myself mainly laughing in agony over Mai Zetterling's French accent) or a more serious drama. The scene near the end in the cab is incredibly poignant, when Petula Clark attempts to save her father's feelings by lying to him.
Hugh Williams was a romantic hero in his earlier career, but by 1949 he is pretty far from being love's young dream. Combined with Mai Zetterling's atrocious accent and the fact we know she is taking him for a ride, there's no real romance to be found here.
Some interesting parallels may be found with Girls' Dormitory (1936) where schoolmaster Herbert Marshall is seduced by French schoolgirl Simone Simon. Marshall is at least single in that one, and Simon's accent is happily authentic. Both films feature a scene in a hut in the middle of a storm, where Hugh/Herbert "rescues" Mai/Marie. There is surely a deliberate parallel or influence there.
As others have mentioned, the spanking scene at the end is frankly outrageous (though amusing for being so) and it's amazing it got past the censors. The silhouetting reminded me of the scene in Seinfeld episode "The Contest" where George visits his mother in hospital. It's very reminiscent of a Carry On film as well.
Unlike many reviewers have claimed: it isn't the case that this student-teacher relationship would be "illegal" in the UK these days. Mai's character is 18 and therefore legally able to consent - the "position of trust" legislation enacted in 2001 does not apply to legal adults. Hugh would doubtless be sacked, but he wouldn't face criminal charges. Unethical, and unwise, but not illegal.