Learn French: Les invités sont censés rester debout. - The guests are supposed to remain standing.

Even though être censé translates to the set phrase "to be supposed to," censé acts like an adjective in French. This means it must agree in gender and number with the subject. Since Les invités is plural, censés takes an s.

Do I need a preposition like de or à between censés and rester?

No, the structure is être censé followed directly by the infinitive of the action verb. You don't use any prepositions in between, which is why it is simply sont censés rester.

I have seen the word sensé spelled with an "s". Is that just an alternative spelling?

No, they are two different words that sound exactly the same! Censé (with a "c") means "supposed to" or "expected to," which is what we use here. Sensé (with an "s") means "sensible" or "reasonable." When you mean "supposed to," always use the one starting with "c".

Why doesn't debout have an s to agree with the plural Les invités?

In French, debout (standing) is classified as an adverb, not an adjective. Because adverbs are invariable, it never changes its spelling to match the gender or number of the subject. It stays exactly the same whether one person is standing or a hundred.