Archives are a wonderful source for teachers. Original documents are the 'raw material' of history. They provide pupils at all stages with an immediate, direct connection to the past, and bring history to life.
Not all archives are able to offer this service. Smaller archives in particular will be restricted in the educational services they are able to provide by space and staff time.
Your first step is to contact the relevant archives repository - see a list of all archives here, and discover whether the one you are interested in is able to offer an educational service.
Some are not able to accommodate visits by pupils, but may be able to offer INSET days for teachers. Where no specific service for teachers is offered, teachers are always welcome to visit archives to look at relevant material. Most archives will be able to make copies for teachers to use in the classroom.