You've already received plenty of sufficient answers for your generic Can I cast Slow on an Invisible Target question, but you also ask a hypothetical about a school bus which hasn't received an answer.

#You could not do this with your school bus hypothetical

The other answers focus on the need (or lack thereof) of line-of-sight. Some spells do, some spells don't, but the default answer is "does not require." This is not the case with line-of-effect, where "does require" is the default. In the scenario that prompted this question, the line-of-effect is clear: you could draw a straight line between you and your target without intersecting anything else, invisible or not. There are no obstacles, and there are no people in the way.

What would stop your Slow from hitting its target is full cover, which blocks line-of-effect. D&D has never provided a great rule about when you're in full cover and when you're not, but Jeremy Crawford has, somewhat controversially, said that glass (such as a school bus window) provides full cover. You could not cast Slow into a school bus:

A solid obstacle, regardless of material, can provide total cover. A closed window counts.

If you were talking about a school bus with, say, shattered windows, it's down to a DM call: what level of cover does this mass of innocent school children provide? There is no clear answer.

You've already received plenty of sufficient answers for your generic Can I cast Slow on an Invisible Target question, but you also ask a hypothetical about a school bus which hasn't received an answer.

You could not do this with your school bus hypothetical

The other answers focus on the need (or lack thereof) of line-of-sight. Some spells do, some spells don't, but the default answer is "does not require." This is not the case with line-of-effect, where "does require" is the default. In the scenario that prompted this question, the line-of-effect is clear: you could draw a straight line between you and your target without intersecting anything else, invisible or not. There are no obstacles, and there are no people in the way.

What would stop your Slow from hitting its target is full cover, which blocks line-of-effect. D&D has never provided a great rule about when you're in full cover and when you're not, but Jeremy Crawford has, somewhat controversially, said that glass (such as a school bus window) provides full cover. You could not cast Slow into a school bus:

A solid obstacle, regardless of material, can provide total cover. A closed window counts.

If you were talking about a school bus with, say, shattered windows, it's down to a DM call: what level of cover does this mass of innocent school children provide? There is no clear answer.

You've already received plenty of sufficient answers for your generic Can I cast Slow on an Invisible Target question, but you also ask a hypothetical about a school bus which hasn't received an answer.

You could not do this with your school bus hypothetical

The other answers focus on the need (or lack thereof) of line-of-sight. Some spells do, some spells don't, but the default answer is "does not require." This is not the case with line-of-effect, where "does require" is the default. In the scenario that prompted this question, the line-of-effect is clear: you could draw a straight line between you and your target without intersecting anything else, invisible or not. There are no obstacles, and there are no people in the way.

What would stop your Slow from hitting its target is full cover, which blocks line-of-effect. D&D has never provided a great rule about when you're in full cover and when you're not, but Jeremy Crawford has, somewhat controversially, said that glass (such as a school bus window) provides full cover. You could not cast Slow into a school bus.

If you were talking about a school bus with, say, shattered windows, it's down to a DM call: what level of cover does this mass of innocent school children provide? There is no clear answer.

You've already received plenty of sufficient answers for your generic Can I cast Slow on an Invisible Target question, but you also ask a hypothetical about a school bus which hasn't received an answer.

#You could not do this with your school bus hypothetical

The other answers focus on the need (or lack thereof) of line-of-sight. Some spells do, some spells don't, but the default answer is "does not require." This is not the case with line-of-effect, where "does require" is the default. In the scenario that prompted this question, the line-of-effect is clear: you could draw a straight line between you and your target without intersecting anything else, invisible or not. There are no obstacles, and there are no people in the way.

What would stop your Slow from hitting its target is full cover, which blocks line-of-effect. D&D has never provided a great rule about when you're in full cover and when you're not, but Jeremy Crawford has, somewhat controversially, said that glass (such as a school bus window) provides full cover. You could not cast Slow into a school bus:

A solid obstacle, regardless of material, can provide total cover. A closed window counts.

If you were talking about a school bus with, say, shattered windows, it's down to a DM call: what level of cover does this mass of innocent school children provide? There is no clear answer.