School history books shouldn't have to tell whether or not the subject is biased, because all history is biased anyway. History is always, ALWAYS, written by the people who win, and these people always, ALWAYS, have a vested interest in the way they are perceived. This is why most American history books tend to downplay the atrocities of the early American settlers, as well as American warfare losses, while highlighting every US achievement and victory.
All history is written with a bias. That is a given. Such a warning would only be necessary if you are highlighting an outlier. It is like the ridiculousness of putting 'careful, its hot' on a coffee cup. Yeah... We didn't know coffee is hot.
Have you ever seen unbiased history? History is generally written by the victor, and the good and bad are interpreted in the age they are read. Atrocities in holy texts can be looked at now as barbaric, when at the time of the writing they were seen as totally reasonable, even more tolerant than the norms of the time. Look at the tact of people examining stories of the settlement of north America these days! They are basically saying all of the people back then should be considered horrible people because we 'know better' now, completely ignoring the context of the time.
No, all history is written with some sort of bias. If you need a disclaimer for that, you likely shouldn't be in a history class.