This is a good question.
I am wondering if Symbolon would be good for high schoolers. I have been going to an Adult Formation program where Symbolon has been used, and I like it. One purpose of Symbolon is to reach the pew sitting Catholics who never really learned their faith in the 70s and 80s (yes, I got the watered down version myself as a kid during this time period). From what I have seen, I haven’t seen anything in the content that would be a red flag for high schoolers.
One thing about confirmation: I could see in a large group of 50 or more 8th graders through 12th grade many kids could come and “coast”, without learning much, or even fall asleep, if they are not expected to internalize and remember. My confirmation program in the 1980s was more of a “well, if you show up at the Monday night classes” and “you take part in the mandatory service projects” you got confirmed. Another thing too was a few kids were given money or a gift after they were confirmed (I wasn’t) and some of them went because “mom said so” and “your grandparents want you to get confirmed.”
Some dioceses have taken confirmation out of the junior high program and moved it up to high school, because many kids confirmed in 8th grade seem to not participate in high school youth. I’ve also seen kids having to come to a youth group as attendance to get confirmed. In one sense, it’s bribing the kids to come to youth group, but in another sense quite a few kids find after coming a few times they enjoy it more than they thought they would (I know, I was one of these kids who expected to do the minimum and found that I enjoyed being a part of a youth group).
One parish where I have helped with Faith Formation made confirmation a 2 year program, which IMHO is better than a one year program. This parish does confirm in 8th grade, and the Baltimore Catechism was used when I was an aide for 7th grade - and this was last year. I may help again this year.
A parish I regularly attend has used Theology of the Body for Teens, which I think is appropriate, and the girls can be separated from the boys for some of the discussions. Years ago, I helped a little with high school youth ministry, and some nights we would separate the girls and the boys, depending on the topics. I also find in some groups “single sex groups” were better for discussion, because some of the boys would try to impress the girls.
Just my two cents.