This is a question that exercises some. It generally isn’t one of my hills-to-die-on, BUT, you do need to be aware that using these terms can be a real problem for some children.
If your pupils genuinely understand what is meant by ‘short’ vowel and ‘long’ vowel then great, these terms can be useful metalanguage when teaching. However, if they don’t understand them (and children are very good at covering up that they don’t understand something that the teacher seems to take for granted that they know) then they can start to fall behind and become frustrated.
For example, I knew that this language was being used in of my Y3 tutee’s schools, so, as I try to work with what my tutees are doing at school whenever possible, I began to use it too as he happily said he understood it. However, as we tackled e-controlled spellings (e.g., not vs note), I began to suspect that he didn’t. I probed as to whether he understood the concepts of short and long, which he did, and then asked whether the vowel sounds I uttered seemed relatively shorter or longer to him. It was evident that they didn’t however. Even when I tried to exaggerate (cheat) and clipped my short sounds and stretched my long, he could not classify the single letter vowel sounds in this way.
This might surprise you. However, what might surprise you more than a child who is actually quite good with basic phonics and phoneme manipulation not understanding this seemingly simple concept, is the fact that HE is correct. Short/long vowel sounds really aren’t shorter/longer than each other, we, i.e. ‘masters’ of reading, just think they are because of how we’ve learned to refer to them. What did make perfect sense to him however, and very quickly became useful, was referring to ‘short’ sounds as ‘first’ sounds and the long as ‘second’.
This often works because children learn the short sounds first and, if taught well, they are the first sounds they think of when they see the single vowel letters. They learn the ‘long’ sounds second, or to be more accurate (as some children, especially in the USA learn the letter names first) the ‘long’ sound should become the second sound they think of when they see an individual vowel letter. Calling them the ‘name’ sounds also works (because they correspond with the letter names).
As a one-to-one tutor, I have the luxury of using the language that makes sense to my individual pupil and often I don’t use any metalanguage of this kind. Class teachers however need to be aware that terms such as ‘short vowel’, long vowel’, that they may take for granted will be easily understood, will not (for good reason) make any sense to some of the children they teach. If they still want or need to use them, they should use them often and consistently and check for understanding as they might with any other abstract terms or labels they might choose to teach. The learning point here is that the labels ‘short’ and ‘long’ as applied to vowel sounds are abstract terms and thus harder to understand/learn than they would likely be if they really were the concrete labels that many adult reading ‘masters’ take them to be. While I may have focussed on just one pupil here, I can attest from my experience that this is not an unusual problem and that many class teachers might be surprised to discover how many of their pupils are confused by this.