Just look at the tutorial.nas file, it already supports stuff like progress/completion messages - and it can be easily extended to also support scoring, high scores or whatever else you may have in mind - making a handful of modifications like these should be straightforward, even if you don't know a thing about Nasal scripting - there are enough folks around here who are happy to lend a helping hand to walk you through, or even post full snippets of code - afterwards, it just boils down to opening existing tutorials or creating new ones.
http://wiki.flightgear.org/TutorialsI suggest to read through these docs, and you'll see that creating simple missions is already supported - even without touching any code, just by editing XML files.
So, that's not a real issue - unless of course you are asking US to create missions for you?
For starters, you could look at existing tutorials and see how they work - start by modifying them, copying snippets around - for example, change messages, numbers, targets - and it will all fall into place pretty quickly. Next, you can start your own little tutorial, such as flying a pattern at KSFO - it really isn't rocket science, and it doesn't require any coding either.