US.

Middle School and Jr. High vary depending on how the grades break.

When I was coming up it was this:

Grade School: 1-6
Jr. High - 7-9
High School - 10-12

But when I hit 9th grade, they changed it:

Grade School: 1-5
Middle School: 6-8
High School: 9-12

Slovakia.

If I am to translate it word-to-word:
Základná škola - Base school (so primary) (9 years) - split into 1st degree (1-4) and 2nd degree (5-9).
Stredná škola - Middle school (2-5 years based on field of study, 4 and 5 year fields are with graduation)

Get ready for a twist: Gymnasium (8 or 4 years) - 8 year gymnasium starts after 5th grade of primary school and 4 year after 9th grade. They also replace middle school. It’s not that popular since you basically won’t get any job without college with gymnasium. It’s really just a preparation for college (literally “high school” (Vysoká škola)) (though perhaps better than middle school).

US. In my dialect, all three are different.

Middle school: 6-8

Intermediate school: 7-8

Jr High: 7-9

I attended an intermediate school that called itself a jr high, so I can understand the confusion.

For my state in Australia its

Kinder Ages 3-5
Prep Ages 5-6
Primary School Ages 6-12, called Grade 1-6
Secondary/High School Ages 12-18, Called Years 7-12

After that you’ve got higher education choices via TAFE or University, theres no cut off ages for that.

School is mandatory from ages 6 To 17.

Finland

0-5 year olds Kindergarten (voluntary)

6 y.o.'s pre-school (mandatory since 2018)

Grades 1-6 in comprehensive school are called “low school” (mandatory) Grades 7-9 in comprehensive school are called “high school” (mandatory)

Second degree years 1-3 (mandatory since 2021), you choose either “upper secondary (gymnasium)” or vocational school (or both).

And if you wish to study further university/uni of applied sciences. Basically everyone does their masters (3+2 years) if they choose uni. Uni of applied sciences is usually 3 years.