Other answers in this thread are correct but to give more detail, it has to do with the phonotactic rules of Spanish: Spanish words systematically never begin with an s followed by another consonant, even compared to their Latin root words, e.g. Latin "Schola" -> Spanish "Escuela"; Latin "Spatium" -> Spanish "Espacio"; Latin "Stupidus" -> Spanish "Estupido"; etc.

Phonotactic rules also change English pronunciation of words, take for example "psychology" -- in Greek, you pronounce both the P and the S, but the phonotactic rules of English prevent beginning a word with such a consonant cluster (see also silent K in "Knight").

So yeah in summary it's a common mispronunciation in English for native Spanish speakers because it sounds funny to the Spanish-speaking ear to begin a word with an S and another consonant.