"On that basis you couldn't have US school textbooks with translations of the letters of Christopher Columbus in them"
I don't know what your point is, most school textbooks don't include Colombus' letters either. And it's not that history textbooks can't include religious content but the more religious content there is the less likely you're going to see it in a secular textbook, grandparent is precisely right.
How on earth do you think you can you avoid including religious content in history textbooks, irrespective of the secularity of the school? You'd have to cut nearly all of it.
Besides I would have thought that a secular school would be one where you would see the widest range of religious content when studying history, as it would be less likely to be biased in favour of any religion in particular.
edit - the following is from a US lesson plan on Christopher Columbus for history teachers in public schools;
"In this unit, students will work with primary source documents written by Christopher Columbus around the time of his voyage to the New World, and with secondary source documents written at a later date. They also have a chance to write their own secondary source material in this unit. Using both primary and secondary source documents within a single unit gives students an opportunity to see the difference between these two types of documents. You may wish to have students work alone or in small groups to read and interpret these documents. Groups could be based on areas of interest (especially for analyzing the scholarly documents), or could allow students with stronger reading skills to help others work through the source documents."
and
"The desire to bring Christianity to native peoples was essential to European exploration. It is a topic that cannot be ignored in discussions of Columbus"
I don't know what your point is, most school textbooks don't include Colombus' letters either. And it's not that history textbooks can't include religious content but the more religious content there is the less likely you're going to see it in a secular textbook, grandparent is precisely right.