What your teacher teaches you is unfortunately a complete nonsense and political propaganda.
There were no "Ukrainians" until the 17 century, and the notion of "proto-Ukrainians"
(as people who lived of this territory) is unscientific.
This is a good example of the use of history for political propaganda.
Very many different peoples lived on the territory of current Ukraine in the past.
Descendants of some of those peoples are called "Poles" now. Where the ancestors of modern Poles "proto-Ukrainians" because they lived on the territory of modern Ukraine ?
Why not "proto-Poles"?
Consider the Jews living in the US now. Most of their ancestors lived on the territory
of modern Ukraine. Were they "proto-Ukrainians"? Then why these Jews are not called "ukrainians"?
There was no Ukrainians before 17 century. And there was no such thing as "proto-Ukrainians". All these are projections to the past of modern political realities.
The distinction between Poles, Ukrainians, Belorussians and Lithuanians was established only AFTER the final partition of Rzeczpospolita in 1795. Actually much later.
EDIT. Let me give two concrete randomly chosen examples. There is a city of Lviv
(Lwow, Lvov, Lemberg, Lemberk) in the current territory of Ukraine. It was founded by prnice/king Danila/Danylo in 13-th century.
According to your teacher this Danylo was a "proto-Ukrainian". But every high school teacher in Moscow will teach that he was "proto-Russian", or simply "Russian"
Now, in 19-th century Lwow was a part of Austria. Should Danilo be called a "ptoto-Austrian" because of this? In the first half of 20-th century Lwow was in Poland. Is Danylo a proto-Pole? What can the other criteria be? Language? You can say that Danylo probably spoke "proto-Ukrainian". But a Moscow teacher who will say that he spoke "proto-Russian" will be equally right. Besides the Moscow teacher will have the advantage of saying that Danila was a blood relative of a Yuri (from the same family) who founded Moscow.
Religion? Danylo was probably brought up Greek-Orthodox. But they say he converted to Catholicism. Is he a "proto-Pole" because of that?
What else permits to quality him as "proto-Ukrainian"? Only the fact modern Ukrainians like him.
Second example. The great poet of 19-th century Adam Mickiewicz. He wrote in Polish,
but his most famous poem (Pan Tadeusz) begins with the words: "Lithuania! My Fatherland!"
He was born and brought up on the territory of modern Belorussian state.
Is he a "proto-Pole", a "proto-Lithuanian", or a "proto-Belorussian"?
Ask you teacher. High school teachers in Warsaw, Vilnius and Minsk will give you 3 different answers: all of them count him as "theirs". (Ukrainians usually don't. But a famous monument to him stands in the center of Lviv).
These little examples are given to show that there is no reasonable notion of proto-Ukrainian. This is a meaningless word. And a high school teacher prescribes a meaning to it, depending on the MODERN political division (or to her own political affiliation).
That's why I called all this business political propaganda.
Suppose that tomorrow Russia invades and annexes the whole of Ukraine (God forbid!!).
Then teachers in Lviv (which will become "Lvov" again), will teach you that "Dan-e-e-la-ah"
was a proto-Russian, (or simply Russian, as they teach in Moscow nowadays).
These examples were related to 13-th and 19-th centuries. But speaking of "proto-Ukrainians" earlier than that is even more nonsensical.
Of course you are not advised to contradict your teacher:-) What I recommend is to use some criticism to what you are taught, and read Sienkiewicz novels, for example, to complement what they teach you at school. Just try Sienkiewicz. You will not regret.
Most of the action is on the territory of the modern Ukraine.
This is a fiction, of course, but this is much closer to history than what they teach in the modern high schools. And then just think yourself: who are his heroes: "proto-Ukrainians", "proto-Poles" or "proto-Lithuanians"?
Who were Adam Kisiel, Jeremia/Yarema Wisniowiecki, Ivan Bogun and Bogdan Khmielnicki?
All proto-Ukrainians? Or some of them were Proto-Poles? And why?