Half violin plot with different factors in R

I have data of a pre and a posttest in two groups: control and treatment. I computed the score and now want to illustrate the differences between groups and tests. As a basis I have a data frame in R containing for each student a row with the results of the pre as well as the posttest:

student_id  group       test_id  Score
145         Treatment   pre      0.12
145         Treatment   post     0.78
109         Control     pre      0.45
109         Control     post     0.99

I have written the following code

s2$test_id <- as.factor(s2$test_id)
s2$group <- as.factor(s2$group)

p <- ggplot(s2, aes(x = test_id, y=score, fill = group)) + geom_violinhalf(trim=FALSE, fill="gray")+
  labs(title="Half Violin",x="Test", y = "Score")+
  geom_boxplot(width=0.1)+
  theme_classic() + scale_x_discrete(limits = rev)
p

producing

Is it possible, that the grey violin can be changed into the distribution of the treatment group (blue) and that I can add a second half violin shape on the left of the pre and posttest depicting the control distribution (red)?

You can use the same grouping variable that you use for the boxplots inside geom_violinhalf, then use the flip argument to flip the correct ones around:

library(ggplot2)
library(see)

ggplot(s2, aes(test_id, Score, fill = group)) +
  geom_boxplot(width = 0.1, position = position_dodge(0.2)) +
  geom_violinhalf(aes(group = interaction(test_id, group)), fill = 'gray',
                  trim = FALSE, flip = c(1, 2)) +
  theme_classic(16)

Data used based on structure given in snippet in question

set.seed(1)

s2 <- data.frame(student_id = rep(sample(500, 50), 2),
                 group = rep(sample(c('Treatment', 'Control'), 50, TRUE), 
                             each = 2),
                 test_id = rep(c('pre', 'post'), 50),
                 Score = runif(100))