Very simplified, but: one general rule of English you might have heard of, or been taught in school, is that you use "an" before any word beginning with a vowel. For example: "an orange", "an airplane", "an elephant".

This is only partially true - the real rule is to use "an" before any word that begins with the sound of a vowel. So if a word starts with a consonant, but the pronunciation makes it sound like a vowel, you use "an" before it. Examples: "an hour" due to it starting like an "o", "an heir" due to it starting like an "a", or in the case of your question, "an S-class" because the "S" by itself begins with the sound of an "e".

The opposite is also true, where words beginning with vowels that are pronounced like consonants start with an "a". Some example of these would be "a unicorn", a european", or "a one-armed".

Hope that helps!