Here is a summary of different metadata standards:

MARC (Machine Readable Cataloging) is a standard that is primarily used in libraries to create bibliographic records for books and other library materials. It has been widely used for decades and is considered a very comprehensive and detailed standard, but can be difficult to learn and work with.

Dublin Core is a simpler metadata standard that is widely used on the internet and for digital libraries. It consists of 15 elements that provide basic information about resources, including title, creator, subject, and date.

MODS (Metadata Object Description Schema) is a standard that is used for describing a variety of information resources, including books, journals, manuscripts, and maps. It is designed to be flexible and adaptable to different types of resources, and is often used in digital library projects.

METS (Metadata Encoding and Transmission Standard) is a standard that is used for encoding descriptive, administrative, and structural metadata for digital library objects. It is often used in conjunction with other metadata standards such as Dublin Core and MODS, and is intended to provide a flexible and extensible framework for managing digital objects.

In terms of which metadata standards are most likely to be used by teacher librarians in schools, this will depend on a variety of factors such as the size and type of the library, the resources available, and the specific needs of the school and its users.

MARC is likely to be used in most school libraries, as it is the standard used in most library automation systems. However, Dublin Core is also commonly used in school libraries for digital resources, and MODS and METS may be used for more complex projects or specialised collections. Ultimately, the choice of metadata standard will depend on the specific requirements and goals of the school library and its users.