In our school or university we sometime sign our friends attendance sheet in the intention of helping them. Now is this a big fault like giving false witness? As per my sense it is not harming others so it may be a small sin. But some of my friends say it’s a big sin like false witness. What Islam says about it?
Your interesting question can be considered from two different aspects. The first aspect as you accurately and nicely mentioned, can be related to the issue of witness, Wallahu-a’lamu. Actually unreal testifying which can has a big sin. (Although there are more major sins about the witness in different events, but anyhow it can be considered as a haram and unreal testifying which you ought not to do it…). Of course asking such question shows that how much it is important for you not to do sin. Well done. God bless you.
It can be surveyed from another aspect. In truth, it can be counted as a sort of lying as well. Because lying solely is not related to saying something by mouth, actually every act of you which express the unreal things can be considered as a lying.
The Prophet (pbuh) said:
Lying decreases the daily bread (or Rezq, as provided by Allah) of the human.
Imam Sadiq علیه السلام (as the 6th Imam of Shia) said:
Allah makes the forgetfulness (lack of memory) dominant on the liars that …
Although it is not at the same level of major sins by itself, but it can be considered as a big sin as well (especially it includes unreal testing as a witness too). Therefore as a consequence it never seems worth of while by doing like that, dear good curious mate.