If the girl is wearing lace panties and the boy a pair of slacks and briefs on can sperm go through the clothing and get the girl pregnant?

Depends: As long as the panties & the slacks are worn in their normal upright position.

Not likely: If you are clothed, and you do not see the semen come out of the briefs and slacks, then no problem. If the semen has gone through the slacks - you will see a wet spot- and has gone through the panties - again you will see wet area - and it is at the entrance to the vagina, then it may be possible to get pregnant. Not likely, but possible.