Currently, in India, there is High School Leaving Certificate- HSLC or Secondary School Certificate- SSC or 10th Exams.
After the declaration of 10th result , students can apply for various technical and non-technical diploma courses.
High School diploma in offered to class 10 students in various other countries like Canada, USA, etc.
So according to your query, It is generally studied for four years as an over course during standard 9th to 12th. High school diploma is a course which leaves the qualification awarded upon the high school graduation, which also means that the student who has done this course will get a GED certificate Or state authorised high school equivalent certificate before the enrolment in the colleges takes place.
Hope this was helpful. Feel free to ask more query.