I Education Apps Review
Don't know how many more there are like this, but it's a good start. Look to the left menu: app reviews are classed by grade level.
At this writing, the reviews include:
Early Elementary: 31
Late elementary: 31
Middle school: 40
High school: 27
Unfortunately, these aren't app reviews as the title indicates; they're blog posts. Some posts review, some don't. But...persevere! iPads in Science for example, discusses quite a few science apps.