i’m an australian writing something set in an american high school, and i need some clarification about a few things.
In Australia, the equivalent of sophomores would have seven classes (English, math, history, science, drama, music and gym for example). Juniors would have six classes and the remaining line of classes would be a study period. Seniors would have either four or five classes and two to three study lines (or they could do more classes and have less free time if they liked, but hardly anyone does)
Is this the same for people in America? if not, what is different? also, are there any compulsory subjects for various year levels, and what are they? (in my school, in our final year, we’re allowed to do any subject we want, but in other schools, English is compulsory)
thanks 
The number of classes is determined by the individual school, or in the case of public schools, by the school district. In general, upperclassman are required to take fewer classes than underclassmen, but this is not universal.
Each state has graduation requirements for public schools, which vary by state. Schools/districts can impose additional requirements on top of the state requirements.
In general, 4 years of English is required to graduate. In most cases, a certain number of years of math, science with laboratory, social studies (with one year of US History being an almost universal requirement), and the same foreign language are required. Some schools may have additional requirements for arts/PE/religion.
*side note. Specifics would be nice. Just give me numbers (it’s set in Washington state if that’s helpful)
Google a public HS in Washington for their shedule. Frankly your question is way too open ended and there is a ton of variety in the USA, Try to think of the states as countries, rather than expecting homogeneity. Even within states and within school districts, schools behave differently. Private vs public, low SES vs magnet, charter vs charter, etc.