A teen boy/girl is taken from home by the government and put on a spaceship. There is only a snarky robot and a caged feral girl (maybe an alien?) onboard. They try to escape via bedsheet rope. The cover is them climbing down the rope. Publish range of 1950–2000. NOT “Have Spacesuit, Will Travel.”
If it’s any help, I read the book in the time span of 1997–2001, and the book came from the school library of Sacred Heart School in Taftville, CT, USA.
This reminds me of Journey to Terezor, by Frank Asch…but I’m not sure why! I think there is an escape by bed sheet, but I’m not too certain about a feral girl. Still, maybe worth checking out.