Lecteur exists but is restricted to language teaching.

The term “lecturer” has different meanings depending on the country. If this position includes both research and teaching, the translation would be maître de conférences invité. It can be found in official documents. If research is not part of the job, that might be attaché, adjoint or assistant d’enseignement invité, intervenant extérieur¹ or just conférencier.

Regarding winter school, the expression seems to be sometimes used untranslated but we also use école or université d’hiver, just like summer school commonly translates to école or université d’été.

¹_{Thanks to Flying_whale for suggesting it!}