As a former HS science teacher, I was part of a department that outlawed all plant experiments.
You can be proud of yourself. Stifle kids interest in science just because the are not trained and experienced scientist. Who made you a teacher?
Safe speeds for current technology would be only slightly higher than space shuttle speeds especially if interstellar space contains chunks of comet ice.
If this is true... forget your 100s years. Make it 10000s.
It would be pretty cool if life in general could outlive this planet. At the moment all life that we know about will die when this planet falls into the sun.
First we were alone. Other planets? No way. Then we discovered: Hey, other suns have also planets. First we were able to see only the biggest ones. Now we can detect smaller and smaller ones. Planets almost seem to be something very common. Life? I really would not worry, that Earth is the only planet with life.