A lecture can not be
bored
tired
since a lecture is not animate.
A lecture might be
exhausted
the lecture was exhausted
the possibilities were exhausted
which might mean it has come to its end, but
so exhausted
is not possible since this might describe the physical state of a lecture.
The lecture was so dull that almost everyone fell asleep.
The lecture was so boring...
The lecture was so exhausting...
The lecture was so tiring...
The students were so bored that almost everyone fell asleep.
The students were so tired...
The students were so exhausted...
are all possible and correct.
The students were so dull that almost everyone fell asleep.
though grammatically correct, may not make sense logically.