Answer: no. Standard reference descriptions treat when as an adverb or subordinating conjunction, not a preposition: it takes clauses, as in when lunch ends, not noun phrases.
You can test any candidate with a simple substitution: a preposition takes a bare noun phrase, so swap in a phrase like after lunch. Since when takes clauses, as in when lunch ends, and never a plain noun phrase such as when lunch, it fails that test.
One honest caveat: no supplied source was verified for this classification, so a grammar explicitly listing when as a preposition would overturn the working answer.
How to recognize the word classes
Conjunction: when introduces a clause with its own subject and verb. Clue: the word after it opens a full sentence-like unit. Call me when the meeting ends.
Interrogative adverb: when asks about time and stands alone or heads a question. Clue: it can start a direct question by itself. When does the shop open?
Relative adverb: when links a time noun to a following clause. Clue: it replaces in which after a time word. I recall the summer when we met.
Preposition, for contrast: a true preposition takes a plain noun phrase, not a clause. Clue: swap in a noun and the phrase still works. After lunch, we walked home.
The boundary that trips people up
The tempting boundary is the relative use. In the summer when we met, when sits right after a noun, so it can feel like a preposition phrase inside the noun. It is still standardly a relative adverb: it opens a clause, we met, not a plain noun phrase.
A true preposition takes a bare noun phrase, so test by substitution. After lunch works; a swap like when lunch does not. Where that substitution fails and a full clause follows, classify when as a conjunction or adverb rather than a preposition.
One honest qualifier: grammars and dictionaries occasionally disagree about wh-words near this boundary. In standard references, though, the confident answer is no: when is classed as a conjunction or relative adverb, not a preposition. If a trusted grammar or dictionary explicitly lists when as a preposition, that would change the answer.
The practical rule for everyday use: if the word after when could stand alone as a clause with its own subject and verb, when is working as a conjunction or relative adverb, not a preposition.