When my daughter was in 6th grade her middle school placed her in 6th grade honors math. Which sounded ok, until I learned that her school had three options for 6th graders in math: 6th grade honors, 6th-7th grade accelerated, and pre-algebra. Out of 5 classes for her grade level, 1 was taking prealgebra, 2 were in 6th-7th grade accelerated, and 2 were in 6th grade "honors". So, she was in the bottom of the math track. And notice there is no "below grade level" class, or remedial class. So "6th grade honors" math encompassed students working on or below grade level!
The reality of the school system where we live is that is is definitely possible for kids to get a great education... if they are in the top track. People can argue about whether tracking is fair or not, if it is beneficial to all students or to only some students or to no students. But when you are dealing, in the moment, with your own child, and your school has a system which tracks kids into different math levels... in my opinion, the better education comes at the top track. The students are often better prepared, have pushy parents (which can be a good thing in getting good teachers and good instruction), are often better behaved. This isn't just a question of what math program is right for the student right now, it is about getting into a good cohort of students and getting a good teacher.
I can't say what a good online program is for learning algebra. I will say that my very smart daughter seemed to do poorly on math tests because in reality, she really hadn't mastered below grade level math! Basics of 3rd-5th grade math were not solid.
I took her to a place called Mathnasium for 3 hours a week all 6th grade. It was incredibly expensive and took a lot of time. It was not frugal. But they took her back to the missing skills she was unsure of. They gave her a lot of time to master these skills, instead of teaching her one way to solve a problem and then quickly moving on. She developed skill with mental math.
Even though she wasn't working on 6th grade skills, the extra instruction helped her enormously. In 7th grade she placed into the accelerated 7th grade math group (pre-algebra) which I feel is an appropriate track for her.
If your daughter has any underlying math deficits, it would be worth knowing that before spending any time trying to teach her algebra. If you try to master algebra without a solid grasp of the underlying math needed, it will be a very shaky foundation. I was extremely pleased with the curriculum at Mathnasium for our purposes.
(The online programs I have heard good things about are through Johns Hopkins Center for Talented Youth, but there is a minimum test score needed to take these classes.)