How to punctuate groups of school grades: "3,4,5 students and teachers"

What you're describing is a range. But a range is normally expressed with the lowest consecutive number followed by the highest consecutive number. For instance (1) 1–20, (2) 1–5, 7–20, or (3) 1–5, 7, 9–20.

If just writing (composing) the example sentence in the question, it would be handled differently:

The other day, students and teachers from grades 3 to 5 talked about rules for kickball.
The other day, students and teachers from grades 3, 4, and 5 talked about rules for kickball.

The following would not work as original written text:

✘ The other day, 3–5 students and teachers talked about rules for kickball.

It wouldn't work, because the common interpretation is that you're talking about a range of people, not of grades.

But when transcribing speech, there is no ability to correct grammar. The words that are said, in the order they're said, need to be faithfully reproduced.

However, symbols that are used, and the particular spelling of words, can all be changed when representing the speech in text.

One common style guidance is that when you write dialogue you do not use numerals. Instead, you spell out the numbers. This helps prevent ambiguity over what the person is actually saying (how they are pronouncing it).

For instance, the following would not normally be written:

"I hear that 2012 was a good year."

The reason for this is that 2012 can be pronounced in multiple ways, and authors want to communicate exactly how the character is actually pronouncing it:

I hear that twenty-twelve was a good year.
I hear that two-thousand-and-twelve was a good year.

So, following this, if the spoken dialogue about students and teachers were transcribed (or written as dialogue), it would likely look like the following at a minimum:

"The other day, three four five students and teachers talked about rules for kickball."

Once there is that base form, a decision would need to be made about punctuation (if any at all aside from spaces) between the numbers. Since punctuation is not part of speech, how to punctuate it is somewhat subjective.

You can use en dashes in this specific phrase, even though you might not in non-dialogue text, or any number of different notations—including hyphens:

"The other day, three–four–five students and teachers talked about rules for kickball."
"The other day, three-four-five students and teachers talked about rules for kickball."
"The other day, three/four/five students and teachers talked about rules for kickball."
"The other day, three, four, five students and teachers talked about rules for kickball."

There might be some particular style guides that indicate how this situation should be handled, but I suspect there is no general consensus.

In short, consider the following:

• If composing narrative text, the sentence in the question would be constructed differently.
• If composing dialogue, the sentence could be constructed differently, but the numbers would be written out rather than expressed in numerals.
• If transcribing speech, only spelling and symbols can be changed. In this case, since the expression in speech is a little strange, there is no clear guidance on what particular symbols should be used.