I got you...
"School Choice" on its face is an education program where the state provides vouchers to parents that will cover the cost of tuition at a private school near them. Thereby, allowing a parent to send their kid to private school for free if they choose. The idea behind this is if more parents send their kids to private school, the more children will receive a quality education. More children will succeed educationally, go to college, etc.
There are several problematic things about these programs. However, where "School Choice" runs into its biggest problem is this: the money to cover the cost of these vouchers (tens of thousands of dollars annually per student). Rather than raise taxes or pull money from defunct state programs, state governments are redirecting money that would otherwise go to the state's public schools.
Less funding going to public schools to pay for School Choice voucher programs means that the students not lucky enough to receive a voucher will suffer with WORSE public schools. Students in public schools in poorer areas will suffer the most as the budget cuts will have a direct, negative impact on their ability to learn. Sadly, this will be the vast majority of students.
Outdated textbooks, overcrowded classrooms due to teacher shortages, little-to-no technology in the classroom, little-to-no extracurricular activities (sports, music, clubs, etc.) are already major issues in many, if not all, poorer public schools. Expect for these issues to worsen as School Choice programs start to take off.
As I said above, School Choice improves the education of the few, while destroying the education for the many.
I hope this makes things a little clearer for you.