We've just moved from Edinburgh to DC my daughter is in 7th grade at a DC middle school. The Edinburgh primary school gave no grades, just a narrative report and it's been a bit tricky to show what she did or knows. She didn't test well on the math test here because some of the vocab was different but now that she's in math class it's easy. The school just moved her to a higher class when we pointed out how she's performing now. It's tricky but I think you can explain to colleges what went on and what other benefits besides grades your child has gotten from this experience.

There are also some things that are more nuanced. Example: physics taken in 8th or 9th doesn't necessarily translate as a high school credit (for pre-requisite purposes) because it is too conceptual and not quantitative enough to prep kids for AP physics. However, biology taken in 9th grade could be transferred in usually without a hitch.