The license of Schema.org is Creative Commons Attribution-ShareAlike License (version 3.0).
Does this mean that any search engine can use Schema.org to implement structure data?
See the FAQ Under what terms can we re-use this documentation (and schemas, examples, software)?
As noted in our terms of service document, schema.org schemas are made available under the Creative Commons Attribution-ShareAlike License (version 3.0). These terms also apply to the supporting documentation on this site and on our blog, as well as to the markup examples used in this site. The software used for our site is also available for opensource re-use under an Apache 2 license. If you have feedback on these terms please get in touch.
Depending on your meaning of "use schema.org", it might be that your search engine doesn’t have to follow the license.
For example, if you parse HTML documents, find Schema.org markup, and interpret it according to the Schema.org definitions, you don’t have to license/attribute anything.
However, if you include definitions/examples (taken from schema.org) on your search engine’s documentation site, you have to license it under CC BY-SA 3.0 and attribute accordingly.