How old is your DC? Where are they planning to go to university?
In the UK, DC take exams in a wide range of subjects at 16 called GCSEs. They are a 2 year syllabus so it would be hard to join midway. GCSE grades are often used (alongside predicted A level grades) by universities to select candidates. Most universities require GCSE maths and English as a basic entry requirement, alongside their A level requirements. It may complicate UK university applications if your DC is joining the UK education system post 16 without GCSEs.
They then specialise in 3 or 4 subjects for 2 years before taking exams at 18 called A levels. Most schools and colleges (college = 16-18 further education in the UK) have minimum GCSE grade requirements to be allowed to study a specific subject at A level. The A level curriculum builds on the GCSE curriculum. A levels are considered US college level and your DC should get college credit for them.
If your DC is planning on studying in the UK, they will need to consider what they want to study at university when they choose their A levels. Each course and university has their own entrance requirements as to the A level subjects required and the grades in those subjects.
The International Baccalaureate (IB) is offered as an alternative to A levels at some schools. It is also a 2 year syllabus but students study 3 subjects at higher level and 3 subjects at standard level. The 6 subjects must include maths, language and literature (usually English literature in the UK), a foreign language, a science, a humanity, and either an arts subject or a second choice from one of the other groups. Again, UK universities have course specific requirements for the subjects, levels and grades. The IB is also considered US college level and your DC will gain college credit for the IB (more than for A levels). As a PP said, some colleges allow students with the IB to skip the first year of college.