It should really be:
"They wonder how far they can go with their new teacher", or "They wondered how far they could go with their new teacher".
"... people have been thinking [about] how far they can/could really go with them".
It's a more literal meaning than "to understand". In the first example, the children are testing boundaries -- wondering how far they can go [with their behaviour] with their new teacher. In the second, the people are wondering what further uses the new stickers could have.