There's a simple word in English for those big yellow conveyances. We call one of them a bus.
But what's the plural?
According to the sign, it's "busses".
There is a word in English spelled "buss".
It's a synonym for kiss in American English.
The sign looks funny if you know that.
To answer the question on spelling, go check the dictionary.
Even the Oxford shows that the plural of the vehicle may have one "s" or two. But look up in the URL and you will see that Oxford is directing you to a site for American English if you are from an IP address in North America.
The online version of the Cambridge English dictionary notes a difference with the two "s" version being American. Ditto the UK English version of the Oxford, once you hunt around a bit to find it.
The online reference grammarist.com is clear enough:
In 21st-century English, buses is the preferred plural of the noun bus. Busses appears occasionally, and dictionaries list it as a secondary spelling, but it’s been out of favor for over a century. This is true in all main varieties of English.
After bus emerged in the 19th century as an abbreviation of omnibus, buses and busses (the logical plural of buss, an early alternative spelling of bus) vied for dominance for several decades. By the early 20th century, though, buses was the clear winner, and it has steadily become more prevalent. Today, buses appears on the web about 15 times for every instance of busses.
So while the way the English school district has used busses to mean the plural of the student transporter, that isn't wrong - strictly speaking - if we are using American English spelling as the default. It is just very antiquated. And unless, they spell connection with an "x" - connexion - then they really should lop one of the "s" things off their plural form of "bus".
Odds are in this case, that the folks making up the signs got the spelling wrong and didn't check to see what was the most common spelling.
Wonder if anyone at the school has picked up on this yet?