Story involving a mysterious substance and a confusion between a peninsula/gulf

So I think by googling around I have almost certainly figured this out (see below), but posting anyway in case someone else is looking for the same book, and to remove any remaining uncertainty.

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I read this sci-fi story as a kid, must have been the late 80s, and this is all I can remember:

• The text must have been in German (the only language I could read back then); nevertheless I remember the cover having the English words "top secret" on them in a circular design, like stamped.
• I’m fairly certain the protagonist is female, though I may be conflating this with other books I read around the same time.
• The plot revolves around a mysterious substance called "cabrium", "cambrium", "caprium" or some such (definitely starting with ca- and ending with -ium, like a periodic element), which is somehow dangerous, so it has to be wrapped in something. I think it is wrapped in tin-foil, but I may be misremembering this.
• At some point in the story, the protagonist has to do some kind of school test or homework. They think they did fairly well, but actually they didn’t. One of the mistakes they made was giving the definition of a gulf when they should have given the definition of a peninsula.

While writing up this question, I googled around a bit and found "Friday’s Tunnel" by John Verney 1 2. The descriptions I found make it a very likely fit: it has a female protagonist, and "caprium". Moreover, I googled "Freitags Tunnel" (the obvious German translation), and found 3. The book cover does have the words "top secret" on it, just not in a stamp design (but I might simply be misremembering that part).

What leaves some small amount of uncertainty left in me is that none of the plot elements mentioned in 1 or 2 I haven’t already mentioned above, nor the picture on the cover of 3, jog my memory the slightest bit. Also, none of those links mention the homework/test scene.

Can you confirm I’m on the right track here?