This is a kind of experiment to show that external factor also influence plant / other living creature growth. We also can discuss about adaptation, relate it with evolution, sub species / variant diversity.
Kind of seeds that came from one parents. Example : bean seeds
Why we do this experiments with seeds that came from one parents? You should ask students when do this activity.
I think some junior and highschool students can answer this question, related with genetic topics. But in my opinion, seeds that came from one parents will have more same character rather than if we take it from random sample. It's also acceptable to get random sample from one plant species. Pick a same size seeds will be fine.
We like to have an "apple to apple" comparison. As a teachers you should explain a small things like this because it will make students start to follow scientific method to find the fact.
Lets make treatment like this:
A : clear water
B : semi polluted water (example, from semi polluted river)
C : poluted water (example, mixed with some oil / detergent)
1 : humus
2 : sand + soil
3 : soil being damaged by chemical fertilizer (NPK)
In this experiments, mix seeds with A1, A2, A3. The other seeds will be B1, B2, B3 and the others are with C1, C2, C3.
We have to do exactly the same treatments for all objects. It means that we also gave same light intensity, same spray intensity, etc.
After some weeks, look the different between all objects (in this activity it will be 9 objects. But if we did it in groups, maybe it can be 18, 27 or more - and it will make we analyze better).
Which one grow better ? Why ? Make a discussion with the kids...Try to make the topics wider according your speciality...