Tests of Semantic Memory suitable for 10 to 18 year old
This seems difficult for a number of reasons.

First, are you interested in testing the retrieval of pre-existing semantic memories or the ability to form new semantic memories?  It is entirely possible that exposure to nature increases one without the other, so be sure you're testing the one you're interested in (or both, carefully).

Second, semantic memory is highly dependent on environmental exposure (e.g., I know that trains in the Netherlands are blue and yellow, but if you haven't been there, you probably don't), and thus creating an appropriate semantic memory task will vary by demographic (much more finely cut than "10-18yo"), location, etc.  This means that a general test of semantic knowledge is unlikely to be valid over a large set of the population.

Third, semantic memory is extremely susceptible to practice effects (otherwise studying wouldn't work), so figuring out something that can be administered more than once without major order effects is unlikely.  You will probably have to find a task and split it into subsets which can be administered at different times, which will compound your demographic problem above by requiring more items.

If you want to test ability to encode semantic memory, the task in a paper like McKoon &amp; Ratcliff (1986) may be helpful.  If you want to test the ability to retrieve pre-existing semantic memories, something like that used in Patterson et al. (2014) would be more useful.  You may want to repurpose either procedure by substituting scholastically relevant facts for their category recognition and/or new encoding tasks.

Automatic activation of episodic information in a semantic memory task.
McKoon, G., &amp; Ratcliff, R.
Journal of Experimental Psychology: Learning, Memory, and Cognition, Vol 12(1), Jan 1986, 108-115. http://dx.doi.org/10.1037/0278-7393.12.1.108

Semantic memory: Which side are you on?
Patterson, K., Kopelman, M., Woollams, A., Brownsett, S., Geranmayeh, F., et al. Neuropsychologia (Nov 22, 2014). http://dx.doi.org/10.1016/j.neuropsychologia.2014.11.024