who always seem to be okay with doing the cross sign over their bodies when all the school stops for prayers at the start and the end of the day.
In Catholicism, the Catholic begins his day, his prayers, and his activities with the Sign of the Cross: “In the name of the Father and of the Son and of the Holy Spirit. Amen.” Making this symbol essentially affirms that Jesus was crucified, which Allah says clearly in the Qur'an that he wasn't:
....And they did not kill him, nor did they crucify him [..] And they did not kill him, for certain.(Surah 4, ayah 157, part)
Making this sign also affirms the trinity, whereas Allah says:
They have certainly disbelieved who say, " Allah is the third of three." And there is no god except one God. And if they do not desist from what they are saying, there will surely afflict the disbelievers among them a painful punishment. (5:73)
And do not say, "Three"; desist - it is better for you. Indeed, Allah is but one God. Exalted is He above having a son. (4:171, part)
It goes totally against what Allah, our Lord and God, says in the Qur'an.
I'm starting to feel pressured about doing it, but I don't know? They say that it is alright, and doing the cross over your body is just an act of protecting yourself, even if you're not christian/catholic
If they believe that invoking anyone other than or alongside Allah can protect them, if they think Jesus (peace be on him) can protect them, then this is shirk, the greatest sin a person can commit.
it could be a symbol for yourself.
A symbol of what for yourself?
In case he intended to make the sign of the cross, then he is imitating the non-Muslims and this is a forbidden matter. The Prophet sallallaahu `alayhi wa sallam ( may Allaah exalt his mention ) said: “Whoever imitates a people is one of them.” [Ahmad and Abu Daawood]
However, in case he intended to glorify the cross by doing so, then this is apostasy as the scholars defined apostasy by: “A Muslim becomes an apostate by a clear statement of disbelief or a wording or an act that implies it, like throwing the Mus’haf (the Arabic text of the Quran) in the dirt, and wearing a Zunnaar (a wide cloth belt worn by priests around their waists).”
http://www.islamweb.net/emainpage/index.php?page=showfatwa&Option=FatwaId&Id=159447
I think it is pretty clear. It is not allowed. It is forbidden. It is a sin. A serious one, that potentially has implications for the validity of one's Islam. Please do not do it, and tell them to refrain from doing it.