Can I study a Master of International relations right away after high school?

I’m thinking about studying Bachelor of Arts+Master of International relations at the Australian National University for 4 years thanks to their flexible vertical double programme but I’m honestly intimidated by the Master’s level classes I’ll have to take. I’m still in high school and I’ll be an international student and will probably struggle with English so I’d like some advice…
Are Master’s classes way more challenging than Bachelor’s?

Thank you in advance for your replies!