I have taught basic Geometric Algebra to high school students. I have not been able to find any appropriate resources. I would only do this with very able students, as otherwise you are risking some serious confusion which could interfere with understanding more basic concepts. As prerequisites, I would suggest that complex numbers and 2D vectors (addition/subtraction) should be understood. I did not need dot product at the most basic level, but I'll need to introduce it when doing trig with GA.
The most difficult concept for students to accept was non-commutative operations. Don't underestimate how big this change is for students. You may be able to demonstrate this with a non-commutative group (the symmetries of a 2D shape including rotations and flips), but this is a rather big detour. You can show how rotating a rubiks cube is non-commutative. I emphasised that they are not allowed to take any shortcuts in rearranging the order of multiplied vectors - just follow the rules:
$$\hat x \hat x=1$$
$$\hat y \hat y=1$$
$$\hat x \hat y= -\hat y \hat x$$
I revised how vectors can be written in terms of basis vectors: $(3,4) = 3\hat x +4\hat y$, and how addition/subtraction works.
Did some simple vector multiplication to demonstrate that distributive and associative laws still work fine, and to practice the anti-commutative property of basis vectors.
I then gave them 3 example vectors: $(1,0),(3,4),(0,1)$ and discussed where they would rotate to. They then had to experiment to find what multiplication could rotate any vector $90^\circ$ anti-clockwise, ie.$(1,0)\cdot \Box = (0,1)$. This was a too hard given the small amount of practice that they had, so I gave them some hints. A demonstration of how $\hat x \hat y$ works can be given (ie. the first $\hat x$ cancels the $\hat x$ of the vector, and then multiply by $\hat y$ to get the desired result), then show it automatically works for the others.
The final step is to ask "what is $(\hat x \hat y)^2$?". After 30 seconds a triumphant student exclaimed "It's $i$!!" (referring to $\hat x \hat y$). Which was of course the whole point of the exercise, to demonstrate why $i$ works - because of the underlying unity of geometric algebra. I did not discuss how complex numbers are confusingly used to encode both rotations and vectors.
I have not introduced dot (symmetric) products or areas and wedge (anti-symmetric) products, though I will touch on this when we get to Trig II. You can easily calculate the cos and sine of the angle between vectors. It is quite beautiful when studying sine and cosine rules - especially the sine rule becomes obvious.
Not a curriculum I'm afraid, but I hope it helps.