If translated literally, 學校的前面是花園 in English could be "School's front is a garden" , and in this case, if we say "front" is "something" ,now you can see 的 here means something(or somebody)'s something. For example, "my pen" in Chinese is " 我(somebody) 的('s) 筆(something). But back to your question, 的 is actually not needed, cause "front" is apparently not "something", so you can also say 學校前面是花園, and for another example 學校(的)後面是山坡。

的 is not necessary here. But the reason it IS here is because the 前面 belongs to the 学校.

потому что без 的 нужно употребить глагол 有

"学校的前面是花园" uses 的(of) to make the noun学校(school) and the other noun前面(front) together. In this way "the front of the school" becomes a new noun as a subject. All the sentence becomes the form "A is B". If you don't want to use 的, 学校前面是花园 is also readable. Because we know 前面 means a direction, even omit the 的(of), we still know this means the front of the school.

Another way to think of 的 is as the English word "of," as in "in front OF the school." (It's another variation of the possessive. The other way you could say it is "the school's front.") Either way, you have to think of 前面 as an object that belongs to the school.