Listen to a Russian who grew up during that "communist period" o'yous, went to school in Moscow during the fifties and sixties, had been caught smoking in school bathrooms and issued reprimands for that.
In on-the-job or in-the-Party situations it was"vygovor" (a serious business, that), but at school it was "zamechanie" (literally "a remark" but in a sense of reproof) that they may also enter into your report book. However, I would translate both as "reprimand".

"Warning" (as someone suggested) is "preduprezhdenie" – a much more serious business, that: it would be issued after a dozen of reprimands, would be pregnant with suspension from school and would require some official reaction from the culprit's parents. So, stick with "reprimand"
In iItalian context, there has never been a communist period – not even close; but they behaved in exactly the same way during the fascist period – so, the same difference....

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Note added at 1 day17 hrs (2009-11-07 15:35:01 GMT)
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Scusa, quale "libretto del partito"? Che frottola è questa?
1. Schoolgirls non potevano essere membri del Partito: esso fu una cosa per gli adulti e, a differenza di ciò che si pensa, abbastanza esclusivo.
2. Potevano essere membri della "Gioventù Comunista", si, ma questo ti dava solo una tessera; e non si può mettere i punti di demerito nella tessera!
3. L'unica soluzione è questa "this reprimand will be entered in your Communist Youth Union record"... a brutta cosa, a proposito: poteva pesare un sacco nel futuro.