Objection 1. It would seem that scandal is a mortal sin. For every sin that is contrary to charity is a mortal sin, as stated above (24, 12; 35, 3). But scandal is contrary to charity, as stated above (A2,3). Therefore scandal is a mortal sin.
Objection 2. Further, no sin, save mortal sin, deserves the punishment of eternal damnation. But scandal deserves the punishment of eternal damnation, according to Matthew 18:6: "He that shall scandalize one of these little ones, that believe in Me, it were better for him that a mill-stone should be hanged about his neck, and that he should be drowned in the depth of the sea." For, as Jerome says on this passage, "it is much better to receive a brief punishment for a fault, than to await everlasting torments." Therefore scandal is a mortal sin.
Objection 3. Further, every sin committed against God is a mortal sin, because mortal sin alone turns man away from God. Now scandal is a sin against God, for the Apostle says (1 Corinthians 8:12): "When you wound the weak conscience of the brethren [Vulgate: 'When you sin thus against the brethren and wound their weak conscience'], you sin against Christ." Therefore scandal is always a mortal sin.
On the contrary, It may be a venial sin to lead a person into venial sin: and yet this would be to give scandal. Therefore scandal may be a venial sin.
I answer that, As stated above (Article 1), scandal denotes a stumbling whereby a person is disposed to a spiritual downfall. Consequently passive scandal may sometimes be a venial sin, when it consists in a stumbling and nothing more; for instance, when a person is disturbed by a movement of venial sin occasioned by another's inordinate word or deed: while sometimes it is a mortal sin, when the stumbling results in a downfall, for instance, when a person goes so far as to commit a mortal sin through another's inordinate word or deed.
Active scandal, if it be accidental, may sometimes be a venial sin; for instance, when, through a slight indiscretion, a person either commits a venial sin, or does something that is not a sin in itself, but has some appearance of evil. On the other hand, it is sometimes a mortal sin, either because a person commits a mortal sin, or because he has such contempt for his neighbor's spiritual welfare that he declines, for the sake of procuring it, to forego doing what he wishes to do. But in the case of active direct scandal, as when a person intends to lead another into sin, if he intends to lead him into mortal sin, his own sin will be mortal; and in like manner if he intends by committing a mortal sin himself, to lead another into venial sin; whereas if he intends, by committing a venial sin, to lead another into venial sin, there will be a venial sin of scandal.
And this suffices for the Replies to the Objections.
Source: Summa Theologica (New Advent)