I need to find a book where a Kid is adopted and he finds out through a Letter he received at his home. He goes on a field trip with his classmates

[Captinraymen]

And the teacher takes the students behind a rock into an elevator thing and they walk out but they have been gone for like 6 years I think

 

[tinkerdan]

Where was the field trip to?
Was it a known place on earth--somewhere on another planet--some alternate fantasy world?
Who was the narrator?
Was it the Kid--was it a number of the students--or just an Omniscient narrator?
How long was the field trip?
Was it short yet they ended up 6 years in the future?
Was it long and unexpected and they were lost?

If it was short and they ended up in the future?
Was the field trip the adventure?
or
Was the adventure when they reached the future?
Or
Both?

Was his being adopted important to what happens on the field trip and the Time jump?
Was his teacher somehow involved in the mystery of the field trip--since he finds the rock that has the elevator?
Were there chaperones on the field trip--such as one of the narrating character's parents?

Honestly I only know of one novel off hand that might or might not fill the few boxes you have there.

Amazon.com

But based on that one that I know, there might be some answers in my qustions that will help someone else who has the answer.