Not Necessarily : possibly but not certainly —used to say that something is not definitely true •Seats in the front row are not necessarily the best. •“We're going to lose this game.” “Not necessarily.” (From Merriam-Webster.com) The examples above show that you can deny the possiblity of something that is likely to happen by using "Not Necessarily". Which means using "必ずしも～ない" as "Not Necessarily" would be preferable since both phrases have almost the same meaning. Good news, though! It's not like you always have to use "必ずしも～ない". I would say both "だから学校のクラスメイトが友達になるわけじゃないです" and "珍しいことは変なわけじゃないです" are definitely correct. But it's slightly better to use "必ずしも～ない" when you want to emphasize the fact that something common may not be true. I hope this helps.

Because dictionaries say that the word "not necessarily" is translated like that.

Thanks a lot! That's very helpful. But I don't understand why you use both 必ずしも and ことではない/わけじゃない. It should change the meaning of the sentence, right? I might write another notebook entry on that topic later.