diagnostic tests in highschool?

Relax. It's not even counted so you can breeze through it without even worrying. Just answer the ones you know and if you don't know it, make a good guess. You'll learn most of the stuff eventually.

I don't really believe these tests. The majority of teens will forget a lot of each of their taken courses over the summer.

Just skim through your notes again and you should do fine.
I took French for 3 years (in high school). ;p

Also remember the golden rule of language tests- lie your ass off if you do not know a word or the sentence construction.
Can not remember the word for (pet) spider- you have a cat
Can not remember the word for brown hair- your [other] has blonde
Can not remember the word for action films- you like romance films
And so on and so on. This even extends to more exotic words- a noun is usually a noun (although remember French has genders for words so lie if you do not know that for a word), an adjective is an adjective and a verb is usually a verb unless it is irregular so I dare say stick with the common stuff even if it is a complete fabrication- you might like paragliding but in the test you like watching films (not to mention you can describe films a lot more easily and in far more depth than some of the more exotic stuff).

Any "creative" writing aspects of a test will almost invariably be counted above the listening or reading sides of things and nailing such a section will probably no see you called out if the other parts drag you down- the way most low level languages are taught you can be a parrot and pass but the creative stuff is where they try and sort people out.

FAST, that's not a bad trick, but it falls to pieces if they give you a picture to analyze.


EDIT: I remember when I took the CPE exam a while back. I got top grades on all segments, except speaking. The speaking/interview/oral exam, whatchamacallit. Writing? Aced it. Listening? 100%. Use of English? Flawless (at the time, it really was). Interview? I froze. I forgot everything, ended up calling the examining lady old, really old, unimaginably old, muttering something about how it's good to have sedentiary hobbies because at that age, you're too weak to walk, so it's good to have something you can do without having to stand up.

...yeah.

The subject was pottery, by the way.

And yes, there was a picture to analyze. It was of a bunch of pots.

.