Here are some factoids that may shed light on the art supplement/portfolio process:
Like the OP’s D, my D attended boarding prep school. She did not plan to major in art in college although it is a big part of her life. We went back and forth many times about whether she should submit a portfolio. She was applying exclusively to small LAC’s and needed substantial FinAid. Her EC’s were somewhat minimal.
I gently pushed for a portfolio to demonstrate a special interest and significant investment of time. Neither her guidance counselor nor advisor gave any input till pretty late, when her advisor looked at the list of colleges she was applying to and said Oh Yes you should do a portfolio. That was the point that D bought into it.
Her guidance/college counselor worked as best she could with the number of students she had, but it was not very personalized. I disagree with other posters who say, oh let the counselor do her job. I think you will find many references here on cc that the school counselors even at very good schools, only have so much time. There is room for families to be actively involved. And I am VERY happy with her school counselor. Some of the best things she did were to use her connections to make inquiries when I was getting nowhere, to suggest schools, and to write a thorough and amazing letter of recommendation, utilizing parental input. But I think my involvement helped create our good results. Some of D’s classmates suffered from poor selections of colleges, no good safeties, etc. These were kids whose parents were not part of the process.
Back to the portfolio:
D used art work that was already done, but it had to be photographed, scanned, etc. Her advisor, who was also one of her art teachers, helped a lot with that. I did some photographing and scanning and tweaking of scans to make them presentable, Pencil sketches do not scan well! Lots of artwork does not fit on home scanner beds! This is definitely a major project even if all the artwork is complete.
Different colleges have different requirements for supplements/portfolios. Maybe at some places you can just submit a few things any old way, but most of her colleges were very specific about format and content. Some wanted a specific number of pieces, one required a couple drawings from life, some wanted an emailed pdf, some you had to submit through their portal (uploading piece by piece). Some required that each piece have a title! (D hated that)
For my D, I feel it was worthwhile but of course there was no control group in this experiment. We were satisfied with her admissions offers and FinAid. One college replied with a very personalized letter from a professor commenting on her portfolio and that happens to be where she is now attending.
So, if desired, a portfolio/supplement can be done outside of AP Art or any special class, if suitable artwork already exists. It will still take some time and work. However, I am unclear if OP’s D has any recent portfolio-worthy artwork or if it is all from middle school. I’m no expert but my hunch is that there would need to be fairly recent art for the portfolio.