Can I re-enter the UK having this type of Visa (valid until January 1, 2019) for a 12-day tour?
Your course is over therefore the premise for your Short Term Study visa is no longer valid. Seeking to enter the UK on a short term study visa where the course is finished gives Immigration the right (but not the obligation) to remove you under Paragraph 320 of the rules. https://www.gov.uk/guidance/immigration-rules/immigration-rules-part-9-grounds-for-refusal (1) the fact that entry is being sought for a purpose not covered by these Rules.
If you want to enter the UK as a tourist you’ll need to apply for a Standard Visitor visa.