compulsory if something is compulsory, you must do it because of a rule or law
Maths and English are compulsory for all students.
Compulsory education was introduced in 1870.
Wearing a seat belt is compulsory.
It is now compulsory for anyone claiming state benefit to register with a job centre.
obligatory [not usually before noun] if something is obligatory, you must do it because of a rule or law. Obligatory is more formal than compulsory
It is now obligatory for all competitors to wear face protectors.
Evening dress is usual, but not obligatory.
Military service is obligatory for all men between 18 and 27.
mandatory if something is mandatory, you must do it because it is the law. Mandatory is more formal than compulsory and soundsstronger
Drug smuggling carries a mandatory death sentence.
In some countries, wearing helmets is mandatory for all cyclists.
not compulsory
optional if something is optional, you can choose whether to do it or not
You don’t have to do French – it’s optional.
The holiday price includes optional excursions such as an evening cruise on the lake.
General Studies is optional for sixth-form students.
voluntary done because you want to do it and not because you have to – used especially about things that you do because you think they will help other people or be useful
The programme is entirely voluntary, and no one will be forced to join.
compulsory schooling/education• A child who lives in a state that requires school attendance must attend some acceptable school during the years of compulsory education.• Parents usually keep their children at home for a couple of years after they have completed their six years of compulsory education.• The State advances two primaryarguments in support of its system of compulsory education.• The use of education services has become more equal during the years of compulsory schooling.• This will help pupils to develop a personallove of reading which will continue after compulsory schooling.• Are students exempt from compulsory schooling or from certaincourses in the curriculum on religious grounds?• The law which makes Work Experiencepossible for young people in their last year of compulsory education specifically forbids their receivingpayment.
From Longman Business Dictionary
compulsorycom·pul·so·ry /kəmˈpʌlsəri/adjectiveadj
something that is compulsory must be done according to a law or rule
The company has introduced compulsory overtime for all its workers.