Below is the corrected text and my explanations:
Nowadays, higher education is no more the privilege of lords and princes. This means that, more and more, young people become students. This short period of time is very important for any individual - it's when his or her future life will be defined. Nevertheless, almost any ex-student speaks of his college years with admiration, and even some sadness. It's not rare to hear a man telling his son about his student life as a perfect period when all [his] problems seemed to be so far away. But is it really true?
1. "Higher education" does not require a definite article ("the")
2. "More and more" is a clause, and needs commas to separate it from the rest of the sentence
3. "and even some sadness" is a new clause, and needs a comma. (just I did here with this explanation)
4. "as of" should be "as"
5. "all the problems" Better if it more personalised with "his", or you can dropped the article "the" entirely:
"all his problems" or "all problems"
Sure, I, myself am just a student and can hardly know anything about my future and the thoughts I will have on the past. However, I already know certain general laws of the universe. In my humble opinion, people remember their school years with such a profound regret; not because these years are the best, but because they have misbalanced their life by constant partying and a lack of labor. Moreover, a human being sees the past as something which is far more beautiful than the present. We have a lot of things we don't really need, but once have we lost them, we begin to appreciate what we'd had. [The reason for this is our instinct that tells "to stock" but the life is always about "to lose".] According to my own experience, I can say that my studies are, and have always been, really hard. I have to spend a lot of time learning new material, which is not always interesting, simply because I hope my future to be easier and brighter than it possibly may be. So, why regret anything that was so rough? We'll never have another life to live. Thus my message is very plain: each day is an excellent day, and shouldn't you spoil it with any useless regrets about "the good times".
1. "I am myself" is acceptable, but a more native speaker would probably say "I, myself, am just a student"
2. "hardly can I" should be "can hardly"
3. "already know" rather that "know already"
4. "In my humble opinion" introductory phrase set apart with a comma
5. "remember their school years"...remove the "of"
6. Semicolon after "regret"
7. "Furthermore" is an alternate word to "moreover" as it will be more understood, but either is correct here.
8. "The reason for this is our instinct that tells "to stock" but the life is always about "to lose"" I know what you are trying to say here, but this construction is rather clumsy. Try this:
"The reason for this, is that our instinct tells us to "preserve", but life is about "loss".
9. "I can say that my studies are, and have always been," needs commas.
10. "which is not always interesting" separate phrase with commas
11. "So, why to regret" don't use infinite here.
12. "We'll never have another life to live, thus my message is very plain, each day is an excellent day and shouldn't you spoil..."
Run-on sentence with commas. Rewrite this way:
"We'll never have another life to live. Thus, my message is very plain: each day is an excellent day, and shouldn't you spoil..."
Built in 1956 and opened in 1973, the Sydney Opera House is one of the most renowned buildings in the whole world. The most famous part of this building is certainly the shell-like roof, construction of which was a serious challenge. Two hundred feet from top to bottom, this splendid combination of tensioned steel, ceramic tiles and huge shells resembles a ship in the harbor and harmonizes with the environment. Experts claim that this building is an example of a certain architecture school, but it's not definitely true. Even though influences of organic and international schools are obvious, we can hardly say that it belongs to any current [???]. Its modern concept is common for the International School, but its shape isn't. It harmonizes with the environment (a trait of the Organic school), but the aesthetic function is more important. Even though it's constructed in cooperation with engineers it's not completely a high-tech school product. The conclusion is quite simple: any object unique in its kind is always a combination of different features.
1. Better to spell out a number at the beginning of a sentence
2. Comma after "bottom", as this is a descriptive clause.
3. Subject-object matching. "Influences" should be plural, since you are talking about "schools" (plural), and
you should then change "is" to "are" to match "influences".
4. "belongs to any current". Any current what? Current school? Current thought? Replace the [???] with the appropriate word. You probably mean "school" here.
4. "hardly can we say" should be "We can hardly say", as "hardly" modifies "say"
5. "high-tech" not capitalised.
6. "And the conclusion is quite simple" Don't start this phrase with "And" and set it off from the actual conclusion with a colon.
I hope this helps.