The Time-Thief by Patience Agbabi
It's midsummer's day and thirteen-year-old Elle and her Leapling classmates are visiting the Museum of the Past, the Present and the Future.
But on the day of the school trip, disaster strikes, and the most unique and valuable piece in the museum, the Infinity-Glass, is stolen! And worse still, Elle's friend and fellow Infinite, MC(2), is arrested for the crime!
To prove his innocence Elle must leap back centuries in time, to a London very different from today. Along the way she will meet new friends, face dangers unlike any she has ever known, and face an old enemy who is determined to destroy her.
Can Elle find the missing Infinity-Glass and return it to its rightful home before it's too late?