What is the plural form use of "tardy" such as: number of tardy arrivals to school, work, etc.? Word spell check does not have any suggested words in this instance.
an occasion when you are late for a class, or a record that a teacher keeps of this
The plural is tardies.
See also Wiktionary:
tardy (plural tardies)
(US) A piece of paper given to students who are late to class.
The teacher gave her a tardy because she did not come into the classroom until after the bell.
Before turning to the dictionary I had posted the following:
As to saying that tardy is an adjective, well it is also used as a noun. Someone can have one absence and one tardy this week. Or someone could have two or more tardies.
We can indeed convert adjectives to nouns by using only the first part of an "adjective-noun" combination. Consider smiley face. That became smiley. Oh, my gosh this girl likes to use smileys in her texts.
If a word is used as a noun, it's a noun.
Tardy is an adjective. There is no plural.
Your sentence ("number of tardy arrivals to school") is correct.
While tardy as an adjective can't be made plural, you can use tardiness to talk about more than once instance of being late.
Your tardiness is becoming a problem. Please show up to meetings on time.
In the US, tardy is most commonly associated with grade school students. There are some instances of turning "tardy" into a noun, such as:
At Casey County Middle School in rural Liberty, Kentucky, Principal Terry Price says, “We had as many as 40 tardies per class period in the morning." Source
In this sense, I think the principal was shortening 'tardy students' to 'tardies' but it could also be 'tardy slips', or notices that a student was late to class. In contexts other than talking about students, some explanation of tardies might be necessary, because it is not standard English. I would be reluctant to talk about a group of adult coworkers as tardies - it seems a little bit disrespectful to talk about them like they are children.