When it comes to teacher/professor recs, there are differences of opinion and different teachers handle it differently.
One thing that never changes: the applicant should ALWAYS waive their right to see it. I refuse to write a letter for anyone until they’ve waived their rights (I send them a tactful email explaining how it works and then they waive).
Some letter writers opt to show the letter to the applicant anyway, totally outside of the system. Some teachers show all the students their letters. Some only show the letter to select students.
It’s uncommon at the HS level, uncommon at the undergraduate level, and increasingly common as you go up academic levels.
As a prof, I’ve showed LORs to only 2 of my undergrads (for med school). They had done research projects with me for several years, and I wanted them to know how great I thought they were. I emailed them so they didn’t have to read it in front of me. They both were very touched by the letters and told me so later. They of course got in because they are awesome.
When I write letters for PhDs who are applying for postdoctoral positions or other jobs, that’s when I usually start showing the letters. This is common in my professional field. Almost everyone shows their letters to job applicants at that career stage (and higher). It ends up being a nice gesture and it’s so validating to hear the nice things people say about you at that stage.
If I were a high school teacher, I would not show students their letters. Maybe if I felt like a student really needed to see it to make an actual difference in their life, I would consider it once in a blue moon. If my son’s teacher offered to show him his letter, I’d tell him to decline. But I don’t think it’s an automatic bad thing, just not what I would do.